Magmatic processes that generate chemically distinct ...

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs Chad D. Deering, Thomas A. Vogel, Lina C. Patino, David W. Szymanski & Guillermo E. Alvarado Contributions to Mineralogy and Petrology ISSN 0010-7999 Contrib Mineral Petrol DOI 10.1007/s00410-011-0670z

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Author's personal copy Contrib Mineral Petrol DOI 10.1007/s00410-011-0670-z

ORIGINAL PAPER

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs Chad D. Deering • Thomas A. Vogel • Lina C. Patino • David W. Szymanski Guillermo E. Alvarado

•

Received: 13 February 2011 / Accepted: 5 July 2011 Ó Springer-Verlag 2011

Abstract Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for Communicated by T. L. Grove.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-011-0670-z) contains supplementary material, which is available to authorized users. C. D. Deering (&) Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA e-mail: [email protected] T. A. Vogel
L. C. Patino
D. W. Szymanski Department of Geological Sciences, Michigan State University, East Lansing, MI 48824, USA e-mail: [email protected] L. C. Patino e-mail: [email protected] D. W. Szymanski e-mail: [email protected] Present Address: D. W. Szymanski Department of Natural and Applied Sciences, Bentley University, Waltham, MA 02452, USA G. E. Alvarado ´ rea de Amenazas y Auscultacio´n Sı´smica y Volca´nica, Instituto A Costarricense de Electricidad, Escuela Centroamericana de Geologı´a, Universidad de Costa Rica, Apdo. 35, San Jose´, Costa Rica e-mail: [email protected]

at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO2, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (*40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the

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thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir. Keywords Juvenile continental crust
Silicic magmatism
Crystal mush
Oceanic plateau
Costa Rica

Introduction The question of how continental crust is generated is fundamental in earth science. Costa Rica is a region where the oceanic plateau of the Caribbean Large Igneous Province (CLIP) and related oceanic mafic complexes have been closely associated with subduction processes for at least the last 25 Ma. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of oceanic island basalts and oceanic arcs. In contrast, younger silicic volcanic rocks are significantly enriched in SiO2, alkalis, and light rare-earth elements (Vogel et al. 2004), similar to the average upper continental crust (Rudnick and Gao 2005). An ideal laboratory to test models of the evolution of silicic magmas that represent juvenile continental crust is NW Costa Rica with its relatively large volume of silicic volcanic deposits ranging in age from less than 8.06 Ma to 0.63 Ma and the absence of preexisting continental crust. Costa Rica has geophysical and geochemical characteristics of continental crust (Drummond et al. 1995; Linkimer et al. 2010; MacKenzie et al. 2008; Sallares et al. 2000; Sallares et al. 2001; Vogel et al. 2004); yet, it consists of the oceanic plateau of the CLIP and oceanic terrains accreted on it (Baumgartner et al. 2008; Rogers et al. 2007). Clues to understanding the transformation of the CLIP to continental crust are found in the silicic volcanic deposits that are abundant in NW Costa Rica. Vogel and coworkers showed that the silicic volcanic deposits of Costa Rica were genetically related to the mafic lavas erupted along the volcanic front. This conclusion was based on the similarity of key trace element ratios (such as Ba/La, Ce/Pb, and U/Th), oxygen isotopes, and radiogenic isotopes in the silicic deposits with those in the mafic lavas along the volcanic front (Vogel et al. 2006). They suggested that the generation of these silicic magmas was due either to melt extraction from partially crystallized, stalled magmas (Bachmann and Bergantz 2003; Bachmann and Bergantz 2004; Bachmann and Bergantz 2008) or partial melting of hot, previously emplaced, crystallized plutons (Tamura and Tatsumi 2002). Partial melting of preexisting crustal material has been proposed by many workers to produce large-volume silicic magmas; however, recent numerical simulations have suggested that producing significant, large-volume silicic melts by this process is inefficient (Annen et al. 2006; Dufek and Bergantz 2005). Partial

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melting of the crust has been well documented (Barnes et al. 2002b; Brown et al. 1995), but there are no known examples of large-scale crustal melting and melt escape features in exposed sections of igneous crust (Bachmann and Bergantz 2008; Barboza and Bergantz 2000; Greene et al. 2006; Hacker et al. 2007; Jagoutz et al. 2007). The purpose of this paper is to evaluate more closely how the physical mechanism of punctuated melt extraction, either from partially crystallized magma bodies or remelting of hot crystallized plutons, has played a role in (1) the evolution and transformation of this oceanic plateau into upper continental crust and (2) producing two distinct silicic magma types (a high-Ti and a low-Ti magma type) through time. Our conclusions are that the Costa Rican upper crust has been converted to continental crust by melt extraction from highly crystalline (50–70 vol.%), calc-alkaline magmas (either from highly crystallized or partially melted plutons). This has occurred under different P-T-fO2-aH2O, which can explain the production of the two distinct silicic magma types. Regardless of the differences in these magma types (produced by different liquid lines of descent), both are similar in composition to the upper continental crust. As conditions have changed in the lower crust through time, first the high-Ti, then low-Ti magmas become dominant. We propose a two-stage process of punctuated melt extraction for the production of evolved melts in the development of juvenile upper continental crust in NW Costa Rica.

Tectonic setting The Central America crust is divided into two main tectonic blocks: (1) the Chortis block to the north, which is underlain by Paleozoic basement and (2) the Chorotega block to the south (Fig. 1a), which has no Paleozoic basement (Hauff et al. 2000a; Rogers et al. 2007). Instead, the basement of the Chorotega block consists of a thick Caribbean Large Igneous Plateau (CLIP) that formed 140–84 Ma (Alvarado et al. 2009) and is related to the Gala´pagos hot spot (Hauff et al. 2000b; Linkimer et al. 2010) and accreted oceanic arcs (Rogers et al. 2007). Costa Rica is located at the southern end of the Middle America Trench with the Cocos plate subducting underneath the CLIP at the rate of 83 mm/year (DeMets et al. 1994) (Fig. 1a). Its tectonic setting is controlled by the interaction of the Cocos, Nazca, and Caribbean plates, resulting in the formation of an active volcanic arc, located about 150 km northeast of the trench. The crust in northwest Costa Rica has been estimated to be about 40 km (Linkimer et al. 2010; Sallares et al. 2001), nearly double the normal thickness of the CLIP (MacKenzie et al. 2008). Significant extension has

Author's personal copy Contrib Mineral Petrol Fig. 1 a Tectonic map of Central America modified from (Rogers et al. 2002). Volcanic centers from the active volcanic front are shown by triangles. The boundary between the Chortis and Chorotega blocks is not well defined (Linkimer et al. 2010). MAT Middle America Trench, EPR East Pacific Rise, CNS Cocos-Nazca spreading center. b Simplified geologic map of Costa Rica modified from Denyer et al. (2003)

(a)

(b) Map modified from Denyer et al.

been documented in Nicaragua, just to the northwest of our study area (Morgan et al. 2008), and a large component of extension has been suggested by (Acocella and Funiciello 2010) for the Central American arc.

Silicic volcanic deposits of NW Costa Rica The silicic volcanic deposits of NW Costa Rica are dominated by ignimbrites (Fig. 1b). There are at least 25

separate units ranging in age from 8.06 Ma to 0.63 Ma (Deering et al. 2007; Gillot et al. 1994; Vogel et al. 2004; Vogel et al. 2007). Their relative stratigraphic position and Ar–Ar ages (Gans et al. 2002; Vogel et al. 2004) are shown in Fig. 2. The stratigraphy is divided into the older Bagaces Formation and the younger Guachipelı´n deposits (Deering et al. 2007) (Fig. 2). Most of these are ignimbrites occur in the broad Guanacaste plateau—a region referred to as the Meseta de Santa Rosa ignimbrite plateau (Chiesa et al. 1987; Gillot et al. 1994). The oldest units (from at least

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Age (Ma) 0.63-0.66

1.31

1.35

Guachipelín

1.18

1.59

2.06

2.96

3.21

Bagaces

1.87-2.1 11

3.64 4.10 4.87

8.06 Fig. 2 Stratigraphic section of silicic volcanic deposits for NW Costa Rica. Ar/Ar ages determined by Gans et al. (2002). Symbols for stratigraphic units are used in Fig. 4c. Gray filled breaks between units represent paleosols and dashed lines unconformities

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8.06–1.87 Ma) belong to the Bagaces Formation and consist of mostly inter-bedded ignimbrites and other pyroclastic deposits, along with an extensive dacitic lava flow near the base (Chiesa et al. 1992; Dengo 1962; Gillot et al. 1994). Recent studies of the Bagaces Formation have documented its stratigraphy in the vicinity of Santa Rosa National Park (Andreas 2005; Mansor 2005; Marquardt 2005) and geochemistry (Szymanski 2007) where it occurs south and west of the Inter-American Highway. The source of the older silicic units is not known and may be buried by the younger volcanic deposits. The younger silicic deposits ([1.47–0.63 Ma) are associated with the Guayabo and Guachipelı´n calderas (Alvarado et al. 1992; Chiesa 1991; Chiesa et al. 1992; Deering et al. 2007; Gillot et al. 1994; Vogel et al. 2004; Vogel et al. 2006)—the precursors to the modern Miravalles and Rinco´n de la Vieja Volcanoes, respectively. These units are dominated by ignimbrites and also include fall and surge deposits (Deering et al. 2007). Originally, the older ignimbrite units (the Bagaces Formation) were thought to have an anhydrous mineral assemblage and to lack biotite or amphibole (Dengo 1962), features used to identify the formation (Dengo 1962) in units west of the Inter-American Highway. However, Szymanski (2007) showed that amphibole and biotite were present in some Bagaces units. Also, units of similar age were recently mapped east of the Inter-American Highway (Arazzi et al. 2004), contain amphibole and occasionally biotite. The youngest ignimbrite units in NW Costa are characterized by abundant biotite and/or amphibole, with rare pyroxene crystals.

Sampling and analytical techniques Glassy pumice fragments (glass ? crystals) extracted from the ash-flow tuffs (glass shards ? crystals ? xenoliths ? pumice fragments), along with minor obsidian samples, were collected during field seasons over 5 years (1999–2004). Glassy pumice samples dominate the samples. Chemical analyses are done on whole pumice samples because their composition is the best estimate of the composition of the magma (crystals ? liquid) immediately before eruption and quenching (Mills et al. 1997). Majorand trace-element whole-rock chemical analyses were carried out at Michigan State University. A total of 452 whole-rock samples were analyzed by X-ray fluorescence (XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Pumice samples were trimmed with a water-cooled diamond saw and/or grinding wheel to remove weathering rinds or visible signs of alteration. Trimmed fragments were rinsed, placed in a sonicator for *5 min, removed, rinsed, and dried in an oven (*60°C) overnight. After drying, samples for

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chemical analysis were placed in a Bico Chipmunk crusher to obtain chips less than *2 cm, if necessary. These fragments were then powdered in a Bico UA Pulverizer alumina flat-plate grinder to a grain size of \50 lm. Sample powders were then placed in a vacuum oven at *60°C for at least 12 h. Homogeneous glass disks were produced for each sample by fusing each powder with a lithium tetraborate (Li2B4O7) flux. All samples were produced using a low dilution fusion (LDF) suited for trace elemental analysis using 9 g Li2B4O7 and 3 g rock powder, with weighing precision of ±0.0005 g. Approximately 0.5 g of ammonium nitrate (NH4NO3) was added to each sample to ensure oxidation of iron during fusion. Samples were weighed in Pt crucibles (95% Pt, 5% Au) and suspended over an oxidizing flame (*1,000°C) on an orbital mixing stage for 25 min. Resulting melts were poured into redhot Pt molds (40 mm diameter) and cooled on a hot plate at *500°C. These glass disks were then analyzed for major elements and selected trace elements by X-ray fluorescence (XRF) and for trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Fused disks were analyzed for major elements (SiO2, TiO2, Al2O3, Fe2Ototal 3 , MnO, MgO, CaO, Na2O, K2O, and P2O5) as well as Cu, Zn, Rb, Sr, and Zr on a Rigaku S-MAX XRF (samples analyzed 1999–2003) or a Bruker S4 Pioneer XRF (samples analyzed 2004–2009) at Michigan State University. Samples were analyzed on the flat side of the disk in contact with the bottom of the Pt mold. A set of 12 international rock standards was used in the calibration. Data were reduced using fundamental parameters (e.g., Criss 1980) with XRFWIN software (Omni Scientific Instruments, USA) on the Rigaku S-MAX and with SPECTRAplus software (Bruker AXS, Germany) on the Bruker S4 Pioneer. Precision is similar for analyses carried out on both instruments, with precision for most major elements \1% RSD, except for TiO2 and P2O5 (\3%) on the S-MAX and P2O5 (\2%) on the S4 Pioneer. Precision for XRF trace elements were less than 10% for Cu and Zn and less than 5% for Rb, Sr, and Zr. In all XRF runs, a minimum of two international standards were measured as unknowns. LA-ICP-MS trace element concentrations were determined on the opposite side of the same glass disks using a CETAC LSX 200 Plus Nd:YAG (266 nm) laser on a Micromass (now Thermo Electron Corporation) Platform ICP-MS with hexapole collision cell. Samples were ablated using a single line scan, after a pre-ablation, to clean and roughen the surface. A spot size of 250 lm and a 150 lm/s scan rate were used for the pre-ablation, while the ablation for data acquisition used a 200 lm spot size, a scan rate of 10 lm/s, and a defocus into the sample of 50 lm.

Using the selected ion recording (SIR) function in the MassLynx software (Waters Corporation, USA), data for twenty-three (23) elements were acquired: V, Cr, Y, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Pb, Th, and U. A set of 12–15 external rock standards, representing a wide variety of igneous rock compositions and prepared using an identical fusion method, was used for calibration. Before processing, the background signal from the Ar/He carrier gas was subtracted from each sample and standard. Strontium, determined by XRF on the same glass disks, was used as the internal standard for quantification. Final concentrations were determined using a standard linear regression for each element (normalized signal vs. concentration), using only standards with calculated values within 15% of preferred, published values. Precision for all elements is \5% RSD. In all LA-ICP-MS runs, a standard treated as an unknown was analyzed. Glassy pumice samples are generally secondarily hydrated by meteoric water and may contain up to 6 wt% secondary H20, which results in major element totals being as low as 94 wt% (Jezek and Noble 1978; Taylor 1968). We arbitrarily used 96 wt% major element totals as our cutoff limit, and samples that had totals lower that 96% were eliminated from our data base. Samples used in our analyses have been normalized to 100%. In addition, four samples were selected for Pb and Sr isotope analyses—two high-Ti samples and two low-Ti samples. These samples were analyzed by Esteban Gazel according to the methods outlined by Gazel et al. 2009.

Chemical variation of silicic volcanic deposits The silicic deposits of NW Costa Rica occur in two major compositional groups, a high-Ti and a low-Ti group, with no overlap between evolutionary trends in the two groups (Fig. 3; representative data are provided in Table 1 and the entire data set in an electronic supplement). The high-Ti group ranges in SiO2 from 58 to 74 wt% SiO2, and the lowTi group ranges from 64 to 79 wt% SiO2 (Fig. 4a). As a group, the high-Ti samples are more mafic, as shown by its higher average Fe2O3(t) content. With one exception, the younger units associated with the Guachipelı´n deposits contain pumice fragments with only low-Ti compositions; the one high-Ti unit is the lowermost unit of the Guachipelı´n deposits. In contrast, the older units (Bagaces Formation) contain both high- and low-Ti compositions, but are dominated by the high-Ti—and generally lower SiO2— compositions (Fig. 4a, b). The individual eruptive units contain either high-Ti or low-Ti pumice fragment compositions, but not both (Fig. 4c). The occurrence of biotite and/or amphibole is restricted to the low-Ti group;

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(a)

(b)

0.9

0.7

0.7

0.5

0.5

0.3

0.3

0.1 50

55

60

65

70

75

80

1

3

SiO2

however, pyroxene may occur in either the high- or low-Ti groups (Fig. 4d). Some other key element variations are shown in Fig. 5. The FeO*/MgO of the high-Ti group is typically greater than the low-Ti group at a given SiO2, although there is some overlap (Fig. 5a). These trends are similar to those produced in experiments on basalt to basaltic andesite at different oxygen fugacities (Sisson et al. 2005); the liquids produced under low oxygen fugacity conditions follow a more tholeiitic trend and those produced under high oxygen fugacity conditions follow a more calc-alkaline trend. With respect to P2O5, the high-Ti group has a near linear variation with SiO2 and ranges from 0.5 to\0.1 wt% P2O5, whereas in the low-Ti group P2O5 is scattered with most of the samples containing \0.1 wt% (Fig. 5c). In general, for a given SiO2 content, Al2O3 is lower in the high-Ti group than the low-Ti group (Fig. 5b). In the low-Ti group, Zr decreases in a broad pattern with increasing SiO2, although the data are scattered (Fig. 6a). In the high-Ti group, Zr generally increases with SiO2, but there are some exceptions where Zr decreases (Fig. 6a); there are two groups of samples in which the Zr decreases with increasing silica. In the low-Ti group, both Sr and Y decrease with increasing silica (Fig. 6b, c). For the high-Ti group, Sr decreases with increasing SiO2, whereas Y is constant (Fig. 6b, c). There is considerable overlap of the REE patterns of the low- and high-Ti groups, as a group, the low-Ti group contains samples with a distinct middle rare-earth element (MREE) depletion (Fig. 7a, b). In considering the highand low-Ti groups for the pyroxene bearing only samples (no biotite and/or amphibole), these same characteristics are present, but the low-Ti group samples have a much more restricted range (Fig. 7b). Pb and Sr isotopes were determined for four samples: two each from the high-Ti and low-Ti magma types (Table 2). There are no differences in the isotopic values for the two magma types. 206Pb/204Pb averages are 18.718259, 2r of 0.001835. 207Pb/204Pb averages are 15.531918, 2r of 0.001607. 208Pb/204Pb averages are

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5

7

9

TiO 2

0.9

TiO2

Fig. 3 a Plot of TiO2 versus SiO2 and b TiO2 versus Fe2O3 for all samples in this study. Red triangles are the high-TiO2 group; blue squares are the low-TiO2 group

0.1

Fe2O3

38.376740, 2r of 0.004652. 87Sr/86Sr averages are 0.703944, 2r of 0.000007. The lead isotopes are similar to those from the active volcanic front in northern Costa Rica, whereas the Sr isotopes are similar to middle Miocene to Pliocene samples from Costa Rica (Gazel et al. 2009).

P-T-fO2-H2O conditions In an effort to explain the origin of the compositional groups described above, we gathered both published, and obtained new, mineral chemical data to estimate the magmatic conditions for each group. Pre-eruptive temperatures and oxygen fugacities of the high- and low-Ti magma types were estimated by Fe–Ti oxide equilibrium using the method of Ghiorso and Evans (2008), which has an uncertainty of ±30°C. Magnetite and ilmenite grains in direct contact were preferred for calculations; however, most were not actually touching, but were included in single phenocrysts. Therefore, the equilibrium test of Bacon and Hirschmann (1988) was used to determine the suitability of oxide pairs for temperature, oxygen fugacity estimates. Temperature fO2 estimates of the high- and lowTi magmas are as follows: Tavg. = 895°C, fO2avg. = DNNO ? 1.0 and Tavg. = 774°C, fO2avg. = DNNO ? 1.3, respectively. A summary of the results is given in Table 3. In the low-Ti group, amphibole is ubiquitous and, therefore, the new empirically derived geothermobarometer of Ridolfi et al. (2010) was applied; data are summarized in an electronic supplement. Temperature estimates determined from amphibole can provide additional insight into the thermal history of the magma, as the crystals do not quickly equilibrate thermally during growth—as with Fe–Ti oxides. The T estimates show a range from *940°C to *775°C (see electronic supplement). The Ridolfi et al. (2010) formulation also provides an estimate of the pressure, which ranges between 75 MPa and 250 MPa, corresponding to depths up to *9 km (see electronic supplement). The amphibole from the dacite crystallized in

Author's personal copy Contrib Mineral Petrol Table 1 Averages and standard deviations of low-Ti and high-Ti samples compared to continental crust (Rudnick and Gao 2005) Major elements (wt%)

Low-Ti (n = 350)

SD

High-Ti (n = 100)

SD

Upper CC 66.60

SiO2

73.26

2.20

68.01

3.19

TiO2

0.31

0.09

0.74

0.09

0.64

Al2O3

15.05

1.08

15.64

0.84

15.40

Fe2O3

2.31

0.80

4.61

1.39

4.53

MnO

0.07

0.03

0.12

0.03

0.10

MgO

0.43

0.20

0.95

0.57

2.48

CaO

1.97

0.53

2.94

1.08

3.59

Na2O

2.54

0.38

3.12

0.53

3.27

K2O

3.98

0.45

3.70

0.79

2.80

P2O5

0.03

0.02

0.17

0.09

0.15

Trace elements (ppm) Zn

Low-Ti (n = 350)

SD

32

11

Rb

83

Zr

153

Sr V

High-Ti (n = 100)

SD

Upper CC

62

10

67

16

76

16

82

27

230

48

193

306

66

358

91

320

47

28

68

37

97

Cr

13

20

12

10

92

Y

16

5

34

4

21

Nb

10

2

13

4

12

Ba

1839

188

1710

281

628

La

28.6

5.6

35.1

7.7

31.0

Ce

52.7

10.3

73.3

16.4

63.0

Pr Nd

5.81 19.68

1.63 6.65

8.95 34.36

1.68 5.84

7.10 27.00

Sm

3.61

1.25

7.02

0.91

4.70

Eu

1.06

0.28

1.72

0.19

1.00

Gd

3.51

1.14

6.57

0.80

4.00

Tb

0.51

0.16

0.98

0.11

0.70

Dy

2.53

0.91

5.39

0.66

3.90

Ho

0.54

0.19

1.12

0.14

0.83

Er

1.66

0.56

3.26

0.38

2.30

Yb

2.11

0.62

3.49

0.57

2.00

Lu

0.33

0.7

0.53

0.07

0.31

Hf

3.5

0.3

5.1

0.9

5.3

Ta

1.0

3.4

0.9

0.3

0.9

Pb

13.9

1.2

10.3

3.4

17.0

Th

7.9

1.0

8.4

2.4

10.5

U Eu/Eu*

4.3 0.95

4.3 0.77

1.9 0.08

2.7 0.72

Nb/Ta

10

15

13

Zr/Hf

44

45

36

La/Yb

14

10

16

the highest P-T regime and as pressures and temperatures of crystallization decrease, they overlap slightly with the rhyolite. This trend is interpreted to reflect a general cooling and polybaric crystallization from dacite to

rhyolite. Water contents range from 3.7 to 4.7 wt%, and oxygen fugacity estimates based on amphibole are up to DNNO ? 2.6 (see electronic supplement), higher than that obtained by Fe–Ti oxides.

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-Relative Stratigraphy 0.1

0.3

0.5

0.7

0.9

50

60

70

80

SiO2

TiO2

(d) -Relative Stratigraphy

(c)

0.1

0.3

0.5

TiO2

Discussion Two end-member processes have been widely accepted for the generation of silicic magmas in oceanic arcs: (1) partial melting of previously emplaced, solidified magmas (Clemens and Wall 1984; Pichavant et al. 1988a, b; c; Riley et al. 2001; Smith et al. 2003) and (2) crystal fractionation, along with some assimilation (Bacon and Druitt 1988; Bowen 1928; Clemens 2003; Druitt et al. 1988; Hildreth and Fierstein 2000a; Hildreth and Fierstein 2000b). Numerical models (Annen and Sparks 2002; Dufek and Bergantz 2005) have shown the difficulty in generating significant volumes of hydrous silicic magma by partially melting cold upper crust that is not already near its solidus temperature—even using fertile starting compositions (e.g., pelite). It is important to note that no abundant pelitic lithologies occur in Costa Rica. Interaction of magma with crustal material to produce melt has been widely documented and reviewed (for example see Barnes et al. 2002a; Brown 2002; Brown et al. 1995). However, in northwest Costa Rica, the Pb, Sr Nd, and oxygen isotopic variation in the silicic deposits are identical to mafic lavas from the active volcanic arc and this precludes large-scale melting of the CLIP basement (Vogel et al. 2006). Because of this isotopic similarity, it is not necessary to invoke interaction with preexisting crust (CLIP). In the discussion below, we do not differentiate between partial melting of near-solidus,

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(b) -Relative Stratigraphy

(a)

-Relative Stratigraphy

Fig. 4 a TiO2 variation with stratigraphic positions. Red triangles are high-TiO2 group; blue squares are low-TiO2 group. b SiO2 variation with stratigraphic positions. c TiO2 variation with stratigraphic position showing the variation of individual eruptive units. Symbols show eruptive units and are given in Fig. 2. d TiO2 variation with stratigraphic position showing pyroxene bearing (asterisks) or amphibole and/or biotite-bearing units (pluses). Horizontal line in all figures separates the younger Guachipelı´n units from the older Bagaces units

0.7

0.9

0.1

0.3

0.5

0.7

0.9

TiO2

plutons, and crystal fractionation, as each would produce similar geochemical trends. Brophy (1991) outlined a physical process for effective crystal–liquid separation, where convection ceases when a critical crystal fraction is reached (*50 vol.%) and becomes rheologically locked (Marsh 1981). At this stage, crystals in suspension begin to form a framework structure and convection ceases. Recently, numerical simulations have also shown that the most efficient interval at which liquids are separated from this crystal mush occurs between 50 and 70 vol.%, regardless of the magma composition (Dufek and Bachmann 2010). The lower limit is first governed by the inefficiency of crystal melt separation, and the short time interval the magma spends in the low crystallinity, convective state, followed by the eventual formation of a crystalline framework. The upper limit is governed by the low permeability of this crystalline structure. It is important to note that crystal mushes can be created either by crystal fractionation of primary (possibly contaminated) magmas or by partial melting of previously emplaced plutons. Partial melting of hot, recently crystallized plutons, still near their solidi, is energetically more efficient than partially melting cold crust. In the subsequent discussion, we evaluate the geochemical evolution of the magmas, fixing the melt extraction window at 50–70 vol.% to provide some physical control over the crystal–liquid separation process for our geochemical models.

Author's personal copy Contrib Mineral Petrol 0.5

350

P2O5

0.4

300

(a)

Zr

250 0.3

200 150

0.2

100 0.1

50

(c)

0 19

700

Al2O3

(b) 600

Sr

17

500 15

400 300

13

(b)

200

11

100

8

50

FeO*/MgO

(c)

6

Y

40

Tholeiitic 4

30

2

20

Calc-alkaline

(a) 0 55

10 60

65

70

75

80

SiO2 Fig. 5 Variation of selected elements and element ratios with SiO2. a FeO*/MgO. b P2O5. c Al2O3. Tholeiitic/calc-alkaline discrimination after Miyashiro (1974)

Two magma types in NW Costa Rica In NW Costa Rica, a complete continuum of compositions from basalt to rhyolite is not observed in the ignimbrites and related deposits, but rather two distinct types that each span a more restricted and offset compositional range (Fig. 3). In other words, the full range of compositions (58–79 wt% SiO2) cannot be explained by one single, continuous liquid line of descent, but instead defines two distinct liquid lines of descent, each derived from different crystal mushes with distinct compositions, temperatures, and oxygen fugacities.

0 55

60

65

70

75

80

SiO2 Fig. 6 a Zr versus SiO2. b Sr versus SiO2. c Y versus SiO2. Red triangles are high-TiO2 group; blue squares are low-TiO2 group

Originally pointed out by Christiansen (2005) and later discussed by Bachmann and Bergantz (2008), silicic magmas can be divided into two broad categories, which are similar to those in NW Costa Rica: ‘‘hot-dry-reduced’’ and ‘‘cold-wet-oxidized.’’ In general, the wet silicic magmas occur in subduction zones and the driest magmas occur in hotspots and continental rifts (Christiansen 2005), but there are exceptions. (Deering et al. 2008; Deering et al. 2010) have recently discussed the occurrence of both types in the Taupo Volcanic zone (TVZ), New Zealand. However, the TVZ rhyolite types (and those in Costa Rica) do not extend to the most extreme end-members observed elsewhere (e.g.,

123

Author's personal copy Contrib Mineral Petrol Fig. 7 Rare-earth element plots for the high-Ti group (red) and the low-Ti group (blue). a All samples. b Pyroxene-bearing samples only. Note that even in the pyroxene only samples, the low-Ti samples exhibit a middle rare-earth depletion, which is indicative of amphibole separation. Chondrite normalization factors from Sun and McDonald (1989)

Rock/Chondrite

(a)

(b)

100

10

1

All samples Red = high-Ti group Blue = low-Ti group Ce La

Nd Pr

Sm Pm

All pyroxene-bearing samples Red = high-Ti group Blue = low-Ti group

Gd Eu

Dy Tb

Er Ho

Ce

Yb Tm

La

Lu

Nd Pr

Sm Pm

Gd Eu

Dy Tb

Er Ho

Yb Tm

Lu

Table 2 Sr and Pb isotope values for representative low-Ti and high-Ti samples Sample

Magma type

206

2r error

207

2r error

208

2r error

87

Sr/86Sr

2r error

020703-1A

Low-Ti

18.857843

0.002074

15.534413

0.001864

38.510995

0.004236

0.704086

0.000008

020630-2C

Low-Ti

18.708579

0.001871

15.521865

0.001242

38.355082

0.003452

0.703952

0.000007

040705-5C

Hi-Ti

18.583006

0.001710

15.578096

0.001464

38.334699

0.003642

0.703848

0.000006

010628-3k

Hi-T

18.723609

0.001685

15.493298

0.001859

38.306186

0.007278

0.703891

0.000006

18.718259

0.001835

15.531918

0.001607

38.376740

0.004652

0.703944

0.000007

Average

Pb/204Pb

Pb/204Pb

Pb/204Pb

Analyses provided by Esteban Gazel using methods described in Gazel et al. 2009

T°C

DNNO

Sante Fe-1

923

1.2

Sante Fe-2

908

1.2

Sante Fe-3

932

1.1

Sante Fe-4

921

1.1

Sante Fe-5

868

1.2

Sante Fe-6

874

1.2

Papapagyo-1

834

0.8

Papapagyo-2

903

0.5

Average

895

1.0

Yellowstone: hot-dry-reducing) and are only slightly hotdry-reducing or cold-wet-oxidizing relative to one another. (We use ‘‘hot-dry-reduced’’ and ‘‘cold-wet-oxidized’’ here only for relative comparison of the two magma types.) In the Taupo Volcanic zone, both types are rhyolites and have similar major element compositions, but different trace element compositions, particularly with respect to REE’s. The differences in chemistry between the two rhyolites were explained as resulting from differences in the phenocryst assemblages in the crystalline mushes, which were governed by variations in the T-fO2-H2O conditions. In NW Costa Rica, the two rhyolite magmas have compositions consistent with liquids extracted from two distinct crystal mushes. The evolution of these two magma types from source to surface is examined in the following discussion.

Low-Ti samples Rio Liberia-1

777

1.4

Lower- to middle-crustal magma evolution

Rio liberia-2

776

1.4

Rio liberia-3

778

1.4

Rio liberia-4

773

1.3

Rio liberia-5

775

1.4

Rio liberia-6

781

1.3

Salitral

774

1.4

Sandillal

759

1.1

Average

774

1.3

Table 3 Temperature fO2 estimates of the high- and low-Ti magmas (based on magnetite and ilmenite microprobe analyses from Szymanski (2007) and this study Sample High-Ti samples

Fe–Ti oxide geothermometry calculations performed using formulation of Ghiorso and Evans (2008)

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MacKenzie et al. (2008) showed that the crustal structure and seismic velocity (Vp/Vs) define an overthickened crust (37.9 ± 5.2 km) and a distinct terrane boundary at the MOHO in Costa Rica, which was interpreted to represent the added mass of mafic rock to the lower crust. Their estimates of lower crustal addition range between 10 and 23 km3/km/Ma. Vogel et al. (2006) showed—based on selected trace element ratios, oxygen isotopes, and radiogenic isotopes—that the intermediate to silicic magmas are

Author's personal copy Contrib Mineral Petrol Guanacaste; natural samples 5

K 2O

4

3

2

High Ti andesite to rhyodacite Low Ti dacite to rhyolite

1 60

65

70

75

80

SiO2 Kawamoto, 1996; experiments 2.0

2wt. % H2O; 0.5 GPa 1wt. % H2O; 0.5 GPa 1wt. % H2O; 1.0 GPa

1.6

K 2O

genetically related to the mafic lavas from the volcanic front in NW Costa Rica—either by crystal fractionation or partial melting of hot previously emplaced plutons. In an effort to provide a connection between the early stages of magma differentiation in the lower crust with that occurring in the upper crust, we examine the generation of the two different magma types in the lower crust first. We begin by evaluating how varying P-T-fO2-aH2O conditions in a primitive magma in the lower crust influence the phase assemblage and composition of the intermediate melt that would be expected to ascend to the mid- to upper crust. Experiments on basaltic differentiation under varying conditions provide information on the phases crystallizing and the resultant liquid in equilibrium with those crystals. Differences in oxygen fugacity and the presence of hydrous phases reflect differences in the conditions of crystal growth in the intermediate to silicic magmas of Costa Rica, and these differences are used as a guide for back projecting the conditions under which the mafic magmas crystallized. Experimental work by Beard and Lofgren (1991) and Kawamoto (1996) evaluated the difference in melt evolution for basaltic magmas at mid- to lower-crustal conditions with different starting H2O contents. In both sets of experiments, at crystal contents within the extraction window (50–70 vol.%), the composition of the liquid for the ‘‘dry’’ (*1 wt% H2O) runs is always less evolved (andesite-rhyodacite) and the temperature at extraction is higher compared to the ‘‘wet’’ (C2 wt% H2O) runs (daciterhyolite). In Fig. 8, we compare the liquid compositions produced in experiments by Kawamoto with the natural samples from NW Costa Rica. We also used MELTS models to evaluate the differences in H2O content in melt evolution (Fig. 9) and produced similar results. These results clearly show the higher water content melts are more silica rich in the melt extraction window. Along the liquid line of descent, this effect is first controlled by the higher abundance of spinel in wet-oxidizing experiments and later the appearance of amphibole, both of which result in an increase in the SiO2 content of the liquid, relative to the dry experiments. In the dry experiment, fewer oxide minerals crystallize and phases with higher SiO2 are dominant (pyroxene and plagioclase), resulting in a lower SiO2 in the liquid relative to the wet experiments. In the experiments of Kawamoto, the higher water content experiments had lower K2O than the dryer experiments at the same SiO2 concentrations (Fig. 8). The earlier appearance of K2O-bearing amphibole in the phase assemblage of the wetter experiments, relative to the drier experiments, governs the differences in the K2O–SiO2 trends in the experiments. We realize that the starting composition of the Kawamoto experiments differs from our sample set and we use them only to show the controls of

1.2

Hot-dry reducing

0.8 Cold-wet oxidizing

0.4 0.0

45

50

55

60

65

70

75

80

SiO2

Fig. 8 A comparison of K2O versus SiO2 content between the experimentally derived liquid compositions at different aH2O from Kawamoto (1996) and the natural samples of Guanacaste

K2O and SiO2 melt compositions by pressure and water content. The results of these experiments are consistent with differences observed in the modal mineralogy and major element chemistry of the two distinct silicic magma types found in NW Costa Rica. Therefore, the initial step for magma evolution in NW Costa Rica is consistent with melt extraction from two distinct primitive crystalline mush zones in the lower to middle crust. In relative terms, these reflect two distinct liquid lines of descent: one crystal mush was a high-Ti, high temperature, dry, relatively lowfO2 crystal mush, and the other a low-Ti, low temperature, wet, high-fO2 crystal mush. The limitations imposed on the physical extraction of melt indicate that the range of high-Ti magmas, through high-andesite compositions, and the low-Ti magmas, through dacitic compositions, can be directly extracted from a primitive basalt at depth without extending much beyond the upper extraction limit (Fig. 9; *70 vol.%). However, further magma evolution to higher SiO2 contents requires an additional stage of differentiation, which will occur in the mid- to upper crust. Although pressure estimates are not available for the high-Ti magmas, those for the low-Ti magmas indicate that the dacitic and rhyolitic

123

Author's personal copy

Melt composition (SiO2)

Contrib Mineral Petrol

assemblage of subduction zone magmas (Davidson et al. 2007). The variation of P2O5 in the high-Ti group (Fig. 5c) is due to the liquid being separated from the crystal mush containing variable amounts of apatite.

Dry basalt liquid line of descent NNO; 1 wt.% H2O

68 66 64 62 60

T = 1100-1000˚C

58

Extraction window

54 52 50 0

20

40

60

80

100

Crystal fraction

Melt composition (SiO2)

Cold-wet-oxidizing magmatism

56

Wet basalt liquid line of descent NNO+1; 2 wt.% H2O

68 66 64 62

Extraction window

T = 1020-880˚C

60 58 56 54 52 50 0

20

40

60

80

100

Crystal fraction Fig. 9 MELTS models demonstrating the extraction of intermediate liquid compositions from a crystalline mush within the extraction window (50–70 vol.% crystals) in the mid- to lower crust under varying T-fO2-H2O conditions

magmas indeed resided in the upper crust prior to eruption (see electronic supplement).

The low-Ti-magmas have chemical signatures that indicate that the liquids were derived from magma that was crystallizing plagioclase ? amphibole ? oxides ? biotite ? apatite ? zircon ± sphene ± pyroxene. The steep negative slope of Dy/Yb and increasing La/Yb are both indicative of amphibole crystallization (Fig. 11a, b). Even in the few amphibole-absent samples, there is MREE depletion, reflecting cryptic amphibole separation (Davidson et al. 2007). Yttrium, which behaves similar to the MREE and is also highly partitioned into amphibole, also decreases and provides additional support for amphibole fractionation (Fig. 6c). Plagioclase fractionation is supported by the decrease in Sr with increasing silica (Fig. 6b). P2O5 is extremely low in the low-Ti group samples (Fig. 5c), less than 0.10 wt%, and reflects a relatively large amount of apatite that has crystallized in the mush, prior to liquid separation, in comparison with the high-Ti group. The inference that amphibole, apatite, and sphene were modally significant in the mush is also supported by the relatively large positive Eu anomaly (Fig. 11). Apatite, amphibole, and titanite have a much smaller partition coefficient for Eu than for the adjacent rare-earth elements (Sm, Gd, and Tb), and the partition coefficients are, for these REE, one or two

Upper crustal magma evolution 2.0

increasing tit

Hot-dry-reducing magmatism The high-Ti magmas have chemical signatures that indicate that the liquids were derived from a magma that was crystallizing plagioclase ? pyroxene ? oxides ? apatite ± zircon. Zircon may have been saturated in a few units (see decreasing Zr with SiO2 trends in a few samples, Fig. 6a) that reached dacite-rhyolite compositions, where zircon typically becomes saturated in arc magmas. Plagioclase was the dominant fractionating phase, which is indicated by the overall negative Eu anomaly (Fig. 7) and the variation of Sr (Fig. 6b) and Al2O3 (Fig. 5b). However, the flat Eu/Eu* trend observed in the silicic magma (Fig. 10) indicates that a competition with apatite, which has a negative affinity for Eu relative to the neighboring elements, is balancing the further decrease in Eu in the melt (Deering and Bachmann 2010). An equal proportion of clinopyroxene to plagioclase explains the relatively shallow Dy/Yb and nearly flat La/Yb ratios (Fig. 11a, b), which have been used to indicate the relative proportions of clinopyroxene amphibole in the fractionating

123

Eu/Eu* 1.5

1.0

0.5

0.0 55

60

65

70

75

80

SiO2 Fig. 10 Eu/Eu* where Eu* is the predicted value of Eu based on the concentration of SmN and TbN on a REE diagram. Note that for the highest SiO2 samples in the low-Ti group Eu/Eu* values are generally [ than 1.0. Values for the high-Ti group are \1.0. All chondrite normalization factors from Sun and McDonald (1989). Red triangles are high-TiO2 group; blue squares are low-TiO2 group. Fractionation models show crystallization in 10% increments with shaded areas over the area of punctuated melt extraction between 50 and 70 vol.% crystals

Author's personal copy Contrib Mineral Petrol 30

+amph

La/Yb

increasing tit

20

10

+px

(a) 0 3

Dy/Yb

+px

2

1

+amph

(b) 0 55

60

65

70

75

80

SiO2 Fig. 11 Selected trace element ratios plotted against SiO2. a La/Yb. Note the nearly constant value for the high-Ti group. b Dy/Yb. Note the values for the low-Ti group a generally \1.0. Fractionation models show crystallization in 10% increments with shaded areas over the area of punctuated melt extraction between 50 and 70 vol.% crystals; trace element ratios indicate variation in amphibole involvement in controlling the trace element signature of the resultant melt Davidson et al. (2007)

orders of magnitude greater in apatite and titanite, respectively, than plagioclase [for example see Rollinson, Figure 2, p. 119 (1993)]. The magnitude and direction of the Eu anomaly will depend critically on this competition between apatite and titanite and plagioclase (Deering and Bachmann 2010). Therefore, the crystallization of apatite and titanite in the mush will produce a positive Eu anomaly in the extracted liquid (Fig. 10). Moreover, the oxidation state of the magma plays an important role in the partition coefficient of Eu in plagioclase. In magmas with high oxygen fugacity (i.e., low-Ti, low-T magmas), Eu3?/Eu2? is much higher than the higher T, lower fO2 magma. Thus, the positive Eu anomaly present in the low-Ti group is consistent with significant amphibole ? apatite ± titanite fractionation at relatively high oxygen fugacity. Temporal magmatic evolution in NW Costa Rica The eruptive stratigraphy combined with new geochemical analyses reveals some simple patterns in the NW Costa

Rica magma evolution through time (Fig. 4). Initially, both high- and low-Ti magma types were erupted from 8.06 to 1.87 Ma. Then, only the low-Ti magmas were erupted. The early erupted magmas, before 1.87 Ma, were dominated by high-Ti magmas (Fig. 4). Pyroxene only bearing magmas occur in both the high- and low-Ti magmas, but hornblende and/or biotite are confined to the low-Ti magmas. Some pyroxene-bearing, low-Ti samples occur at the base of the Guachipelı´n group (Fig. 4), but no pyroxene-bearing samples occur in younger samples (1.47–0.63 Ma). Thus, from about 10 Ma to 1.87 Ma, the composition of erupted magmas oscillated between hot-dry and cold-wet types. Similar observations were made by Deering et al. (2008, 2010) for the Taupo Volcanic zone (TVZ). A major difference between the silicic magmas of TVZ and NW Costa Rica is that in the TVZ there is a continuum between the magma types, whereas in NW Costa Rica there is little overlap between the two types. Deering et al. (2008) explained the variation from cold-wet-oxidizing magmas to relatively hot-dry-reducing magmas as being controlled by variations in the fluid flux contribution from the slab producing the cold-wet-oxidizing magmas, followed by accretion at lower crustal levels, producing the hot-dryreducing magmas (see Deering et al. (2008), Figure 21, p. 2273). In NW Costa Rica, the hot, dry magmas dominate the older sequence and the younger sequence contains only the cold-wet-oxidized magmas. Generation of juvenile continental crust Composition of upper continental crust The silicic deposits of NW Costa Rica display the common large ion lithophile element (LILE) enrichment, and high field strength element (HFSE) depletion observed in magmas generated by subduction processes. In addition, the incompatible trace elements of these ignimbrites are similar to the upper continental crust (Rudnick and Gao 2005); they are slightly enriched in the most incompatible elements (Rb, Ba, Th, and U) and depleted in P and Ti (Fig. 12a). This is distinctly different from silicic deposits from oceanic island arcs such as Izu-Bonin (Tamura et al. 2009) and the Marianas (Wade et al. 2005) (Fig. 12b). The compositions of these magmas do not match the estimates for the composition of the upper continental crust (Rudnick and Gao 2005). This is particularly evident in spider plots of evolved magmas from oceanic arcs that have been suggested as potential examples of juvenile continental crust formation. In these plots (Fig. 12b), it is clear that silicic magmas from Izu-Bonin are significantly different than estimates for the upper continental crust, particularly with respect to the incompatible trace elements. Kelemen and coworkers (Kelemen 1995; Kelemen et al. 2003) have also pointed out that for a given

123

Author's personal copy Contrib Mineral Petrol

Rock/Primitive Mantle

1000

(a) 100

10

1

the upper continental crust was in the range of 66–68 wt%, whereas the low-Ti samples from Guancaste are higher. However, in our model, these more silicic compositions represent liquids extracted from the cold-wet-oxidizing dacite, which will leave a complementary cumulate residue. The parental dacite may represent the average composition of the upper continental crust, but the erupted rhyolitic magma with complementary cumulate [low-dacite to high-andesite (Deering et al. 2010)] together will also have an average dacite composition.

Rock/Primitive Mantle

1000

(b) 100

10

1 Rb Th Nb K Ce Pr P Zr Eu Dy Yb Cs Ba U Ta La Pb Sr Nd Sm Ti Y Lu

Fig. 12 Spider plots of silicic volcanic deposits compared to upper continental crust compositions. a High-Ti group (red) and low-Ti groups (blue) of Guanacaste, Costa Rica. b Two oceanic arcs that have been proposed to represent juvenile crust formation. Izu-Bonin in green: data from Tamura et al. (2009) and Anatahan Island, Marianas in purple: data from Wade et al. (2005). The upper continental compositions are plotted as stars and are from Rudnick and Gao (2005). Chondrite normalization factors from Sun and McDonald (1989)

amount of silica, most oceanic arc lavas have Mg# [molar Mg/(Mg ? Fe)] greater than bulk continental crust. To address this inconsistency, researchers have suggested that the addition of ocean island basalt (OIB) components to arc magmas may replicate the bulk trace- element signature of the continental crust (Barth et al. 2000; Rudnick and Gao 2005). This OIB component is present in the Guanacaste silicic samples. Gazel et al. (2009) estimated between 0.5 and 1.0% OIB type component in the lavas from the active volcanic front in northern Costa Rica. Our Pb isotope data for the silicic ignimbrites (206Pb/204Pb = 38.376740, 2r = 0.001607; 206Pb/204Pb = 18.718259, 2r = 0.001835 and 208 Pb/204 Pb = 38.376740, 2r = 0.004652) are similar to those reported by Gazel et al. 2009 for lavas from the active volcanic front (See Gazel et al. 2009 Figure 9A) and represent a similar contribution from an OIB source. Both the high-Ti magma groups and low-Ti groups have identical Pb isotope values. Although the incompatible trace element concentrations in the Guanacaste samples are similar to estimates of the upper continental crust, there are some differences. For example Rudnick and Gao (2005) estimated the SiO2 for

123

Important geophysical conditions for growth of continental crust In addition to the geochemical similarities to average upper continental crust (Rudnick and Gao 2005), geophysical and temporal characteristics provide important clues to understanding the physical controls governing the formation of juvenile crust. Both hot-dry-reducing and cold-wet-oxidizing dacite magmas in NW Costa Rica, similar in composition to the upper continental crust, were erupted at the surface and probably spent very little time stored in the upper crust and largely represent liquid compositions (i.e., crystal poor). However, to induce a second stage of upper crustal dacite storage, which is required to produce a rhyodacite to rhyolite liquid, the dacite must become trapped. The volume of trapped dacite/high-andesite complementary cumulate to the erupted rhyodacite to rhyolite is likely around 5:1 (White et al. 2006), which means the eruption of rhyolitic magmas provides somewhat indirect evidence of a significant volumetric crustal input. Estimates of juvenile upper crustal addition in Costa Rica can be made by using the 5:1 ratio of dacite cumulate to rhyolite melt extracted at the 50–70 vol.% crystallinity interval as an estimate of the efficiency of melt extracted. Using the lower crustal mafic addition estimated by MacKenzie et al. (2008) of 10–23 km3/km/Ma, intermediate magma volume additions to the upper crust are from 640 to 1,472 km3 along 40 km over the 8 Myr time span where evolved magmas began to appear at the surface. There are several physical conditions that promote ‘‘trapping’’ of magmas and are consistent with the progression of eruptions from typical arc-type mafic magmas to the later dominance of silicic magmas and, in particular, the most recent wet-oxidizing rhyolite in the depositional record (Fig. 5): (1) Davidson et al. (2007) reviewed the effect of high water contents on magmas in arc settings as they were emplaced into the crust. As these magmas rose toward the surface, they fractionated substantially compared to water under-saturated magmas (e.g., Carmichael 2002; Davidson et al. 2007), which will ensure they rapidly reach the rheological locking point (*50 vol.% crystals). Although the relatively high H2O contents may

Author's personal copy Contrib Mineral Petrol

be an important factor for providing the optimal conditions for rapid crystallization, it is not a requisite for the development of an upper crustal reservoir. The initially high temperature differential between wall rock and intruding magma and the rapid heat loss (as the magma is convecting at low crystal fractions) ensure that the magma will spend little time at the low crystallinity state (Huber et al. 2009). Once rheologically locked, the decreasing temperature differential with wall rocks, impeded convection, latent heat buffering, and periodic reheating during recharge will tend to keep the reservoir above its solidus, losing heat slowly via conduction (Bea 2010; Huber et al. 2009; Koyaguchi and Kaneko 1999; Marsh 1981). This explains how even the relatively hotter-drier intermediate magmas can also stall in the upper crust to produce rhyodacite to low-silica rhyolite; (2) Deformation of the thick, upper plate by the subducting lower plate produces local extension and strike-slip motion that creates space with pull-apart structures and releasing bends (Acocella and Funiciello 2010; Alaniz-Alvarez et al. 2002; Vigneresse 1995) that allow magma to accumulate and crystallize (Cambray et al. 1995; Hanson and Glazner 1995; Hughes and Mahood 2008; Hughes and Mahood 2011). The temporal transition from mafic to felsic (similar to upper continental crust) magmatism at *8 Ma is coincident with the counterclockwise rotation of the arc in Costa Rica between 15 and 8 Ma and the initiation of extension in NW Costa Rica and Nicaragua (Gans et al. 2002); (3) Thermal maturation of the crust transforms the stress from a brittle to viscoelastic regime, promoting reservoir growth and magma storage rather than brittle failure and eruption (Jellinek and DePaolo 2003). In the incipient stages of transitioning to wetter basaltic magma evolution that produces dacite melt compositions within the extraction window, both the formation of space due to pull-apart structures or releasing bends and the thermal maturation of the crust are particularly important conditions that promote shallower emplacement and trapping of primitive magmas within the lower to middle crust. In NW Costa Rica, these processes have led to the transformation of increasing portions of the oceanic plateau into upper continental crust. Acknowledgments We greatly appreciate our many friends in Central America who made this project possible. We are especially appreciative of the many discussions with the participants of the Margins meetings in Costa Rica that helped focus our work. Olivier Bachmann, Tom Sisson, and Jorge Vazquez made helpful comments on earlier versions of this manuscript that improved the paper and clarified some of our interpretations. CDD was supported by the Royalty Research Fund from the University of Washington. The thoughtful reviews by Calvin Barnes and an anonymous reviewer are appreciated for helping us to focus our discussion, which greatly improved the manuscript.

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