Geological Society, London, Special Publications A Laurentian? Grenville-age oceanic arc/back-arc terrane in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina Graciela I. Vujovich and Suzanne Mahlburg Kay Geological Society, London, Special Publications 1998; v. 142; p. 159-179 doi:10.1144/GSL.SP.1998.142.01.09
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A Laurentian? Grenville-age oceanic arc/back-arc terrane in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina GRACIELA
I. V U J O V I C H 1 & S U Z A N N E
MAHLBURG
KAY 2
I Laboratorio de Tect6nica Andina, Departamento de Ciencias Geol@icas, Universidad de Buenos Aires-CONICET, Argentina. Pabell6n 2, Ciudad Universitaria (1428), Buenos A ires, Argentina (e-mail:
[email protected]) 2Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY, 14853, USA (e-mail.
[email protected]) Abstract" Whole-rock trace element, field, and petrographical studies indicate that the
protoliths of highly deformed metamorphic rocks of the Pie de Palo Complex, Argentina, formed in an oceanic setting like that of SW Pacific supra-subduction complexes (e.g., Lau Basin). Age constraints suggest that this Mesoproterozoic complex, which includes some of the most primitive Grenville-age magmas known on Earth, formed in 80) ratios, are consistent with them all having arc granodioritic protoliths. However, important differences occur among them. In particular, a positive Eu anomaly (Eu/Eu* = 1.24), and high AlzO3 (19%) and Sr (542ppm) concentrations point to PP108 being plagioclase-rich. Abundant plagioclase and a relatively flat REE pattern (La/Yb= 12) indicate low pressure differentiation consistent with evolution in an arc on
oceanic or attenuated continental crust. In contrast, the other samples have lower overall REE concentrations, steeper REE patterns, and virtually no Eu anomalies. The pattern of PP99 (La/Yb = 21), and particularly that of PP98 (La/Yb=44), is consistent with equilibration with a high pressure mineral residua that was plagioclase-poor, as indicated by the lack of an Eu anomaly, and garnet-bearing, as indicated by a steep HREE pattern and low HREE abundances. Such a situation is expected if mafic arcderived magmas equilibrate with high-pressure garnet-granulite or eclogite facies units in a thickened crust. Relatively high Th, U, and Ba concentrations support crustal contamination. Samples PP98 and PP99 are difficult to ascribe to the same arc/back-arc setting on thin crust as the other Pie de Palo Complex units. A solution is for these magmas to have been emplaced in an arc regime superimposed on older units after crustal shortening and thickening by compressional deformation. Such a deformational event could be associated with collision and accretion of a terrane containing the Pie de Palo Complex to a larger continental block. Detailed structural analyses and geochronology are needed to evaluate the geometry and timing of such a possible collision
Discussion: a Mesoproterozoic oceanic arc/back-arc setting for the Pie de Palo Complex All of the protolith types assigned to the Pie de Palo Complex, except that of the Quebrada de! Gato silicic gneisses, can all be found among the arc and marginal basin units of the western Pacific region. This correspondence lends credence to the proposal that the Pie de Palo Complex formed in an oceanic arc/back-arc setting in Mesoproterozoic times. To explore this proposal, the units of the Pie de Palo Complex are compared with those of the Lau Basin region, whose characteristics and evolution have been reviewed by Hawkins (1995a, b). A cross-section across the Lau Basin region after Hawkins (1995b) is shown in Fig. 8 with the proposed Pie de Palo Complex analogues at the base. An important factor in interpreting the protolith of the Pie de Palo Complex is the time required for emplacement of the major units. Existing age constraints indicate that most of the dated Pie de Palo Complex units were emplaced between 1090 and 1060Ma. Including the single zircon fraction 2~176 age of 1118 Ma for the Cerro Barboza schist, the time span is around 60 million years. Interestingly,
GRENVILLE OCEANIC TERRANE IN PIE DE PALO the arc and back-arc development recorded in the Lau Basin-Tonga region spans more than 40Ma (Hawkins 1995b), which is roughly the same order of magnitude. In detail, Hawkins (1995a, b) suggests that the modern Lau-Tonga supra-subduction system began forming in the Early to Mid-Miocene. The Eocene units in the fore-arc formed in older convergence systems that were carried along by subsequent back-arc rifting. These earlier subduction systems were probably located well to the west; their polarity is uncertain. The protolith of the Pie de Palo Complex probably formed in a similarly complex system. A critical factor in comparing the Pie de Palo Complex with the Lau Basin region is the chemistry of the mafic magmas. To this end, some key chemical characteristics of Lau Basin region magmas are compared with those of Pie de Palo Complex samples in the Hf/Th/Ta discrimination diagram of Wood et al. (1979) in Fig. 7. Importantly, amphibolites with N-MORB like light REE depleted patterns from the Quebradas Las Pirquitas/E1 Quemado and Guayaupa plot in the same field as light REE depleted lavas from back-arc ridges in the Lau region (kau Spreading Centre in Fig. 8). The Quebrada Las Pirquitas Fe-rich mafic schists lie on the margin of this field. These samples and the Quebrada Guayaupa amphibolites that fall in the E-MORB field have compositions like Peggy Ridge and Rochambeau Bank magmas in the northern Lau Basin (not shown in Fig. 7; see Hawkins 1995a). Equally important, the Valdivia cumulates (V44, V45) with melt-dominated trace element patterns fall in the field of the Tofua field, which is representative of Lau region arc lavas. Other cumulates from Cerro Valdivia (V43) and Quebrada del Gato (P100) with melt-dominated trace element tendencies, and amphibolites from Quebrada Molle (P10 and P12) and Mogote Corralitos (P3) plot in the arc region near the Tofua field. The only mafic Pie de Palo Complex samples without Lau region equivalents are Quebrada Guayaupa amphibolite P79, which could be a greywacke, and the shoshonitic-like Quebrada del Molle sample P13. Shoshonitic lavas are found in other parts of the western Pacific region (e.g., seamounts in the Marianas trough, see Stern et al. 1988). The Quebrada del Gato and Cerros Valdivia/ Barboza arc cumulates resemble units found on the Lau Ridge. Uplift of such a ridge is consistent with erosion of lavas from the top, exposing the arc cumulate section. The other major Pie de Palo Complex protolith types also occur in the Lau region. The Quebrada Las Pirquitas Fe-rich mafic schists are
175
consistent with hydrothermal venting activity which is common in back-arc rifts. The Pie de Palo marbles are explicable as stromatolitic reefs equivalent to coral reefs under arc-derived volcaniclastic rocks in the Lau fore-arc or on the Lau ridge (Fig. 8). The greywackes are explicable as volcaniclastic sediments shed from an arc split by back-arc rifting (see Clift 1995). Melting of older arc crust is thought to be largely responsible for the dacitic and rhyolitic magmas underlying the volcaniclastic sequence in the fore-arc of the Tofua arc (Fig. 8; Hawkins 1995b). Sample QQ25 whose protolith is interpreted as a dacite could be equivalent to these units. As expected for crustal melts in an arc region and immature greywackes eroded from an arc source, the Pie de Palo gneisses have arc trace element signatures. The only samples that do not fit the Lau region analogy are the Quebrada del Gato silicic gneisses with steep REE patterns. Their trace element characteristics could be explained if they were younger plutonic rocks emplaced in a crust thickened by collision of the Pie de Palo arc/back-arc basin complex with a larger continental block. The above comparison with the Lau Basin region implies that the Mesoproterozoic subducting slab dipped westward relative to the modern Sierra de Pie de Palo as shown in Fig. 8, but the possibility that the slab dipped eastward cannot be ruled out. In the latter case, the Quebrada del Gato/Cerros Valdivia and Barboza units would be in the position of the Tofua arc in the Lau region, and the central and eastern gneisses and amphibolites in the back-arc. Westverging structures in the Sierra de Pie de Palo region could be taken as evidence for this orientation, since west-vergence is the general case over an east-dipping subduction zone. The major structures in the Sierra de Pie de Palo could indicate a Mesoproterozoic collision between a Pie de Palo arc/back-arc complex over an east-dipping subduction zone and a terrane to the west. A problem is that the age of the major structure relative to the arc/back-arc sequence is unknown.
Implications for Gondwana-Laurentian Terrane interactions from the Pie de Palo Complex An important regional tectonic question is the relation between the Pie de Palo Complex and the kaurentian-derived Precordillera basement to the west (Fig. 1). A major Mesoproterozoic tectono-magmatic event is reflected by Grenville-
176
G. I. VUJOVICH & S. M. KAY
Lau Basin Region at 20~ After Hawkins 1995b Extinct Arc Lau Ridge
Tofua Arc Lau Spreading Centre Reef .
~"\~u~'~-
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:
~
LIKE
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-""
.-"
Q. del Gato C. Barboza C. Valdivia
~
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.,.--
......
~
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/
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Old Reef
Remnant EoceneOligoceneArc Complex
with s,icic melts
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Q. Las Pirquitas, Q. El Quemado
Old Arc-derived Sediments
Central and Eastern Pie de Palo
Mesoproterozoic Pie de Palo Complex Equivalents Fig. 8. Cross-section of the Lau Basin region modified from Hawkins (1995b) showing the culmination of >40 million years of arc/back-arc history. All of the protolith types seen in the Mesoproterozoic Pie de Palo Complex are present. The inset at the bottom shows the correspondence of the various parts of the Pie de Palo Complex to the Lau Basin region given a west-dipping subduction relative to the modern Sierra de Pie de Palo. The possibility that the subduction zone dipped to the east also exists. In that case, the Quebrada del Gato, Barboza and Valdivia arc rocks, shown in the position of the extinct arc, could be switched with the Central and Eastern Pie de Palo units. age (c. l l00-1050Ma) crystallization and metamorphic ages in both ranges. A key question is whether or not the Pie de Palo Complex was transported from Laurentia to Gondwana along with the Precordillera Terrane. Key problems in understanding the relation between the Precordillera and Pie de Palo Complex are the age of the ductile thrusting that puts the Pie de Palo Complex over the low grade siliciclastic sediments and limestones of the Caucete Group, and the age and origin of the Caucete Group. The last ductile motion on the thrust is known to be c. 395 Ma (see Ramos et al. this volume), but the history of earlier motions is unknown. Available constraints allow the age of the Caucete Group to be anywhere between Grenvillian and early
Palaeozoic (see Ramos et al. 1996). A further complication is that the region between the Pie de Palo Complex and the Precordillera is mostly covered by Miocene to Recent sediments. To the west, the eastern part of the Precordillera consists of the Villicum and Zonda ranges (Fig. 1) which are largely composed of Cambrian to Ordovician sedimentary rocks. The Lower Cambrian units in these ranges have palaeontological and stratigraphical affinities to Laurentia (e.g., Astini et al. 1995). They are overlain by platform carbonates associated with a passive margin that endured until the lower Ordovician, and then by Ordovician to Devonian sediments that accumulated in a back-arc basin (e.g., Astini et al. 1995, 1996). Palaeontological evidence
GRENVILLE OCEANIC TERRANE IN PIE DE PALO indicates that the eastern Precordillera block was part of Gondwana by the mid-Ordovician (e.g., summary in Dalziel et al. 1996). Evidence for an Ordovician Sierra de Pie de PaloPrecordillera suture is not apparent from radiometric dating in the Caucete Group. It is possible that the strong regional Devonian thermal event recorded by 4~176 white mica ages of c. 395 Ma (Ramos et al. this volume) overprinted an Ordovician event. This Devonian event is generally associated with collision of the Chilenia terrane with the Precordillera to the west (Ramos et al. 1986, 1996). A scenario, which is not mutually exclusive with overprinting of an Ordovician event, is that the Precordillera and Sierra de Pie de Palo blocks were together in the Cuyania terrane (see Ramos et al. 1996) by Early Palaeozoic time, and that the Gondwana-Laurentian suture is on the eastern side of the Sierra de Pie de Palo (e.g., Dalla Salda et al. 1992; Astini et al. 1995; Kay et al. 1996; see Fig. 1). In the latter scenario, Grenville-age deformation and metamorphism could mark suturing of the Pie de Palo Complex with the Precordillera block. The collision would have had to occur on the eastern margin of Laurentia as the Precordillera was attached to Laurentia until latest Precambrian time (e.g., Ramos et al. 1993; Astini et al. 1995, 1996; Dalziel et al. 1996). Subsequently, the Cuyania terrane, composed of the Precordillera and the Sierra de Pie de Palo block, could have rifted from Laurentia to be sutured to Gondwana in the Early Palaeozoic. The most likely site of detachment of the Precordillera is the Ouachita region, east of the Llano uplift in Texas in the United States (e.g., Astini et al. 1995; Kay et al. 1996; Dalziel et al. 1996). Such a scenario is attractive as an explanation for the juvenile arc/back-arc protoliths with Grenville metamorphic overprints that occur in both the Precordillera (Kay et al. 1996) and the Sierra de Pie de Palo, but that are largely missing in the Grenville basement of eastern North America. This hypothesis needs to be tested with constraints on the age of the west-verging structures of the Pie de Palo Complex and the Caucete Group, and the location of the Lower Palaeozoic GondwanaLaurentian suture.
Conclusions Conclusions from this field, petrographical and geochemical study of the units of the Pie de Palo Complex are as follows. (1) Patterns of immobile and some mobile trace elements are useful in establishing the protoliths of the heavily deformed, greenschist
177
to amphibolite facies, metamorphic rocks in the Pie de Palo Complex. (2) The Pie de Palo Complex represents a Mesoproterozoic Grenville-age oceanic arc/ back-arc complex similar to that formed in the western Pacific Lau Basin region. Existing radiometric constraints suggest that the protolith of the Pie de Palo Complex, like Eocene to Recent Lau Basin units, was emplaced in a time frame on the order of 40 to 60 million years. (3) The juvenile ultramafic and mafic magmatic rocks in the Pie de Palo Complex, along with those in the Precordillera basement to the west, are among the most primitive Mesoproterozoic magmatic rocks known on Earth. (4) The mafic sequence in the Quebradas Las Pirquitas/E1 Quemado region represents the basaltic and plutonic section of a back-arc spreading ridge ophiolite in which lavas have N-MORB like light REE depleted trace element patterns and ratios involving La, Th, Ta, and Hf. Mafic Fe-rich schists in the sequence could reflect hydrothermal processes at the spreading centre. Evidence for a back-arc origin comes from association with the other lithological elements expected in a Lau Basin type arc/back-arc complex. (5) The mafic-ultramafic sequences in the Quebrada del Gato and at Cerros Barboza and Valdivia are largely composed of the cumulate sections of an island arc ophiolite. An arc setting is indicated by evidence for primary amphibole and chromite, and characteristic Ta, Th, Hf and REE ratios in melt-dominated trace element patterns. The lack of obvious lava counterparts can be explained by uplift of the extinct arc after it was rifted by back-arc spreading. Uplift could have been accentuated by collision with a terrane to the west. (6) Amphibolite bodies associated with gneisses in the central and eastern parts of the Sierra de Pie de Palo have trace element signatures indicating that a mixed arc and back-arc origin. Most are best interpreted as lavas and cumulates. One has the characteristics of an island arc shoshonite. (7) Intermediate to silicic Pie de Palo Complex gneisses probably largely originated as arcderived sedimentary units (greywacke). Melting of arc crust at the time of arc splitting can account for gneisses with silicic magmatic affinities. Marbles are best interpreted as stromatolitic reefs. (8) Intermediate to silicic gneisses with calcalkaline arc signatures in the Quebrada del Gato are intrusive into the mafic-ultramafic sequence. Trace element evidence for a high pressure mineral residue in some is consistent
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G. I, V U J O V I C H & S. M. K A Y
with e m p l a c e m e n t in an arc crust thickened by a m a j o r c o m p r e s s i o n a l d e f o r m a t i o n . This event c o u l d reflect s u t u r i n g o f the Pie de Palo C o m plex to a n o t h e r terrane. (9) Evidence for juvenile a r c / b a c k - a r c p r o t o liths with Grenville crystallization a n d metam o r p h i c ages in b o t h the Pie de Palo C o m p l e x a n d the Precordillera b a s e m e n t are consistent with a M e s o p r o t e r o z o i c collision o f these terranes a l o n g the L a u r e n t i a n margin. Both w o u l d have been a t t a c h e d to the L a u r e n t i a n m a r g i n o f the m o d e r n s o u t h e a s t e r n U S in the Late Prec a m b r i a n w h e n they were rifted f r o m L a u r e n t i a as the C u y a n i a Terrane. This implies that the L o w e r Palaeozoic G o n d w a n a - L a u r e n t i a n suture lies east o f the Sierra de Pie de Palo as p r o p o s e d in a n u m b e r of p a p e r s (e.g., Dalla Salda et al. 1992; Astini et al. 1995; K a y et al. 1996; R a m o s et al. 1996). Partial funding for this project was provided by the following g r a n t s - US National Science Foundation EAR92-05042, UBACYT Ex-132 and CONICET PID 4162. We thank R.W. Kay, V. A. Ramos, and Eldridge Moores for discussion, J. Abbruzzi and M. L. Gorring for help with analytical work, and M. Godeas, N. Pezzutti and L. Villar for help with petrography. R. Pankhurst, T. Brewer, and especially J. Hawkins provided comments which improved the manuscript. The Argentine Servicio Geoldgico Nacional and Fundaci6n Antorchas supported the field work. This paper is a contribution to IGCP 345 and IGCP 376 'Laurentian-Gondwana Connections before Pangea'.
Appendix: analytical methods Whole-rock samples were pulverized in an aluminium oxide ceramic shatter-box. Electron microprobe analysis for major elements, and Instrumental Neutron Activation Analyses (INAA) for trace elements were carried at Cornell University. Glasses were prepared for major element analysis from c. 1 g of powder fused in a molybdenum strip furnace or fused with a gi metaborate flux in carbon crucibles. Analyses were performed on a three-spectrometer JEOL-733 Superprobe in wavelength dispersive spectrometric (WDS) mode and are based on averages of 4 to 6, 30-ram spot analyses. Operating conditions were 15 kV accelerating voltage and 15nA beam current. Standards were natural mineral and basaltic glasses. Data were reduced on a Tracor-Northern system using Bence-Albee matrix corrections. Typical two-sigma precision and accuracy is 1-5% for major elements (e.g., >1 wt%) and c. 5 10% for minor elements (e.g.,
