Detecting magma-poor orogens in the detrital record G.J. O’Sullivan1, D.M. Chew1, and S.D. Samson2 Department of Geology, Trinity College Dublin, Dublin 2, Ireland Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA
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ABSTRACT The clastic record is commonly interrogated by analysis of detrital heavy mineral assemblages, with the bulk of modern detrital geochronological studies employing U-Pb dating of detrital zircon. However, the bias of zircon toward felsic igneous sources, and the limited ability of the U-Pb system in zircon to record low- to medium-grade metamorphic events, means that the U-Pb detrital zircon record is largely insensitive to magma-poor orogens. In this study, U-Pb ages were obtained by laser ablation–inductively coupled plasma–mass spectrometry for apatite and rutile extracted from alluvium of the French Broad River (FBR) in the southern Appalachians (southeastern United States). In contrast to previously published FBR U-Pb zircon data sets, which yield essentially no record of the most recent Appalachian metamorphic events (ca. 320 Ma) associated with assembly of Pangea, the U-Pb detrital rutile and especially the U-Pb apatite systems together provide a complete record of complex polyphase Appalachian orogenesis. It is unexpected that the apatite and rutile U-Pb Appalachian age populations differ significantly, with probable low-temperature breakdown of rutile biasing the rutile data set toward the most recent (Alleghanian) metamorphic event. These data make the FBR one of the most intensely studied river systems globally for multiproxy single-grain U-Pb analysis, clearly demonstrate dependence of provenance information on mineral proxy choice, and emphasize the resolving power of multiproxy provenance studies.
INTRODUCTION
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Heavy Mineral Provenance Analysis and the U-Pb Detrital Zircon System Detrital zircon continues to be the primary phase employed in U-Pb heavy mineral provenance studies (e.g., Thomas, 2011), due to both the ubiquity of zircon in clastic sediments and because it typically exhibits high U and low common Pb contents, resulting in relatively precise age determinations. However, there are several factors that hinder the ability of the U-Pb detrital zircon system to detect key tectonomagmatic events. These include (1) the refractory behavior of zircon in sedimentary and magmatic systems, meaning that it seldom represents first-cycle detritus (e.g., Meinhold et al., 2011); (2) an inherent bias in the U-Pb zircon record toward zircon-fertile sources such as felsic plutonic rocks (e.g., Moecher and Samson, 2006); and (3) extremely high closure temperatures with respect to Pb (>~900 °C), combined with the limited growth of new zircon below upper amphibolite–eclogite facies conditions in the absence of crystallization from partial melt (see Kohn et al., 2015). These characteristics render the detrital U-Pb zircon system insensitive to tectonic events in orogens that have undergone primarily low- to medium-grade metamorphism, as evidenced by the virtual absence of Alpine metamorphic zircon in the pro- and retro-foreland basins of the
European Alps (e.g., Krippner and Bahlburg, 2013; Mark et al., 2016); to the detrital zircon U-Pb record, the Alpine orogen is effectively invisible. In addition, the studies of Hietpas et al. (2010) and Moecher et al. (2011) on modern alluvium from the French Broad River (FBR; Fig. 1) in the southern Appalachians have established that the U-Pb detrital monazite system preserved a more complete record of Appalachian orogenesis than the U-Pb detrital zircon system, and also retained far less refractory (Grenville) age information. In particular, the detrital zircon record completely fails to record the terminal phase of Appalachian orogenesis, responsible for the formation of the supercontinent Pangea. However, even detrital monazite, although able to record every phase of the Appalachian orogen, does not characterize
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Figure 1. Geological map of the French Broad River catchment in western North Carolina (USA); sample locations are marked (modified from Moecher et al., 2011). BFZ—Brevard fault zone; Cambro.-Ord. Seds.—Cambrian–Ordovician sediments. Western Blue Ridge is here defined as the Great Smoky Group and Snowbird Group.
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those phases by an abundance equal to each event’s importance. Instead, detrital monazite primarily records the Taconic orogeny (the ca. 470 Ma peak in Fig. 2). Monazite is also not as ubiquitous as other minerals; there is a lithological bias toward metapelites. Therefore, less lithologically biased and ubiquitous thermochronometers, which capture the broad range of temperatures commonly encountered in regional metamorphic belts, should also be employed. We utilized the sample suites of Hietpas et al. (2010) and Moecher et al. (2011) to investigate the efficacy of the detrital apatite and rutile U-Pb systems for detecting orogenesis in magma-poor collisional mountain belts. Because their temperature sensitivities (375–550 °C and 490–640 °C, respectively; e.g., Cochrane et al., 2014; Kooijman et al., 2010) are lower than that of monazite or zircon, they are more suited to detecting medium-grade regional metamorphic events in the detrital record. The existing U-Pb detrital zircon and monazite data sets make an ideal framework with which to compare the U-Pb apatite and rutile data generated for this study. The addition of these data makes the FBR one of the best characterized modern fluvial systems in terms of multimethod U-Pb accessorymineral geochronology. Advantages of Apatite and Rutile as Heavy Mineral Provenance Indicators Apatite crystallizes in a much wider range of igneous rock types than zircon (basic through to felsic rock compositions) because the major rock-forming minerals cannot incorporate significant P in their crystal structure (Piccoli and Candela, 2002), and unlike zircon, apatite crystallizes in metamorphic rocks of all grades and most protolith types (Spear and Pyle, 2002). Apatite also more likely represents first-cycle detritus than zircon due to its chemical instability during prolonged periods of alluvial storage in humid climates or extensive weathering at source (Morton and Hallsworth, 1999). Apatite is, however, chemically resilient during transport and diagenesis in temperate conditions. Coupled with work linking the trace element composition of apatite to specific igneous source rock compositions (e.g., Belousova et al., 2002), recent advances in U-Pb dating of common Pb-bearing minerals (Thomson et al., 2012; Chew et al., 2014), and integration with the long-established apatite fission-track and (U-Th-Sm)/He methods, apatite has widespread application as a predominantly first-cycle, triply dateable provenance proxy (e.g., Carrapa et al., 2009) with trace element characteristics that can be linked back to the source rock protolith. Rutile, the most common and stable TiO2 polymorph, is hyperstable in sedimentary systems, helping to define a zircon-tourmaline-rutile assemblage characteristic of the most evolved
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Figure 2. Comparison of detrital U-Pb apatite and rutile data with published detrital U-Pb zircon and monazite (Hietpas et al., 2010) data from the French Broad River (western North Carolina, USA). The U-Pb apatite system clearly records Grenvillian orogenesis to a fuller extent than the U-Pb rutile system. A single U-Pb rim age represents the entire Alleghanian detrital zircon record (Hietpas et al., 2010).
detrital heavy mineral assemblages. Rutile exhibits the unusual combination of chemical and physical stability at both surface and highgrade metamorphic conditions, but exhibits instability at greenschist facies conditions (Zack et al., 2004). In comparison with apatite, rutile crystallizes in a more restricted suite of lithologies (medium- to high-grade pelites and metabasites, especially during high-pressure, low-temperature subduction metamorphism, and some felsic plutonic rocks; Meinhold, 2010). However, rutile generally exhibits higher U content than apatite and is thus usually more amenable to higher precision U-Pb dating. The trace element geochemistry (particularly Fe content and Cr/Nb ratio) of rutile can be linked to derivation from pelitic, metabasic, or felsic-igneous protoliths (Zack et al., 2004; Triebold et al., 2012). GEOLOGICAL BACKGROUND The regional geology of the FBR catchment is complex, and the terrane-scale description here is only a brief synopsis of the relevant orogenic events (for a detailed discussion, see Moecher et al., 2011). The study area comprises the ~12,000 km2 catchment of the FBR in North Carolina and Tennessee (USA). It is an orogen-perpendicular river system that samples detritus from polymetamorphic Laurentian terranes of diverse metamorphic grades, lithological compositions, and apparent ages (both in terms of protolith and
the timing of orogenesis), separated by several major shear zones active throughout the Paleozoic. Dominant lithologies in the catchment of the FBR include felsic and mafic orthogneisses, and metasedimentary schists and paragneisses. As the river flows northwest it bisects the Appalachian chain, sampling terranes that achieved progressively lower peak-metamorphic grade during the Paleozoic, ultimately flowing over the anchimetamorphic rocks of the Valley and Ridge province (Cambrian–Ordovician sedimentary rocks that formed the Laurentian passive margin). The crust in the FBR catchment has undergone four main orogenic episodes that are all detected in the U-Pb detrital apatite and rutile data. These include the ca. 1 Ga Grenville orogeny, which is inferred to have formed much of the crystalline basement in the region; however, Grenville orogenic rocks represent only ~12% of the exposed crust in the watershed. With three Paleozoic orogenies (the Taconic, the AcadianNeoacadian, and the Alleghanian), these four episodes together define the Southern Appalachian Mountains. The Taconic orogeny (ca. 470–440 Ma) formed from the collision of the Taconic arc with the Laurentian margin, resulting in upper amphibolite facies (and local granulite facies) regional metamorphism with extensive anatexis, migmatization, and pluton emplacement. The Neoacadian orogeny (ca. 380–340 Ma) resulted from arc or microcontinent accretion (possibly the Carolina superterrane) to the Laurentian margin. The Alleghanian orogeny (ca. 320–290 Ma) formed as a result of the collision of Laurussia (post-Acadian Laurentia plus Baltica) with Gondwana, forming the supercontinent Pangea. With each successive Paleozoic collision, new terranes were accreted to the eastern margin of Laurentia, stepwise moving the metamorphic front in that direction. However, the regional metamorphic history is complex, with overprinting and retrogressive episodes, although the metamorphic cores of each successive orogeny remain distinguishable (Merschat, 2009). The Neoacadian and Alleghanian orogenic episodes strongly deformed rocks in the region but were not characterized by significant plutonism, and did not attain the peak metamorphic temperatures undergone during the Taconic orogen west of the Brevard fault zone (BFZ, Fig. 1) (Dennis, 2007). Acadian and Alleghanian peak metamorphic conditions were sufficient to either neocrystallize monazite, or reset the U-Pb ages of older monazite, in the western Inner Piedmont (Fig. 1). However, to the west in the eastern Blue Ridge (Fig. 1), the lower Neocadian and Alleghanian peak metamorphic temperatures resulted in minimal new monazite growth. Thus, the majority of monazite ages reflect Taconic orogenesis (Hietpas et al., 2010). A map that places our samples in context of the regional metamorphic
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Figure 3. French Broad River (western North Carolina, USA) detrital apatite and rutile U-Pb data with samples grouped according to geographical distribution and basement affinity. Horizontal scale (207Pb-corrected age) is logarithmic. Alleg.—Alleghanian.
isograds is provided in the supplementary material (Fig. DR1 in the GSA Data Repository1) METHODS Heavy mineral separates collected by Hietpas et al. (2010) were reprocessed using standard mineral-separation techniques to produce apatite and rutile mineral fractions. All grains were analyzed using a Photon Machines Analyte Exite ArF Excimer laser-ablation system coupled to a ThermoScientific iCAP-Qc ICPMS at Trinity College Dublin. Recently developed matrix-matched standards, i.e., the Madagascar apatite, and McClure Mountain and Durango apatite, were the primary and secondary apatite U-Pb age standards, respectively. Primary and secondary rutile U-Pb age standards were the R10, and R19 and RZ3 standards, respectively. Data reduction was undertaken using Iolite 3.1 using the VizualAge_UcomPbine data reduction scheme (Paton et al., 2011; Chew et al., 2014), which accounts for the presence of variable common Pb in the primary age standard and 1 GSA Data Repository item 2016287, Figure DR1 (isograd map), Table DR1 (sample locations), and all apatite (U-Pb and TE) and rutile U-Pb data, is available online at www.geosociety.org/pubs/ft2016.htm, or on request from
[email protected].
dating studies a large initial number of crystals be included per sample in case a significant proportion of analyses yield no useful age data (cf. Mark et al., 2016).
Western Inner Piedmont (FB2, FB3, FB4) The lack of almost any apatite or rutile yielding Taconic (ca. 470–440 Ma) U-Pb ages (Fig. 3A), and the presence of Neoacadian and Alleghanian monazite (Hietpas et al., 2010), corresponds with Neoacadian sillimanite-grade metamorphism in the western Inner Piedmont (WIP; e.g., Merschat, 2009) hot enough to neocrystallize or reset apatite and rutile U-Pb ages. The dearth of Alleghanian zircon in this region (Fig. 2; Hietpas et al., 2010) is consistent with a lack of widespread anatexis. Eastern Blue Ridge and Grenville Basement Units (FB1, FB5, FB6, FB8) An increasing proportion of Neoacadian versus Alleghanian apatite and rutile, compared to the WIP samples (Fig. 3B), is encountered west of the Brevard fault zone. These data imply that during Alleghanian orogenesis temperatures were not high enough to reset Neoacadian (and older) apatite and rutile on a terrane-wide scale in the eastern Blue Ridge (EBR) and Grenville basement (GB) units, in agreement with the absence of pervasive Alleghanian deformation in the region (e.g., Merschat, 2009).
makes it possible to date those U-bearing phases Western Blue Ridge that contain appreciable common Pb. CommonIn the western Blue Ridge (WBR) and the Pb correction in the unknowns employed a start- Valley and Ridge (VR) regions downstream of ing estimate for the age of the unknown, calcu- the internal portion of the orogenic belt, there lating its corresponding 207Pb/206Pb initial ratio is an appearance in the detrital data set of preusing the Stacey and Kramers (1975) terrestrial Phanerozoic apatite and rutile grains (Fig. 3C). Pb-evolution model and then adopting an itera- Significant populations of Grenville-age apatite tive approach based on a 207Pb correction (Chew signify the capture of detritus from source regions et al., 2011). A 207Pb correction means it is not that were not heated above the apatite U-Pb clopossible to exclude U-Pb age data based upon sure temperature (375–550 °C) after the Grendiscordance. All grains with 207Pb-corrected 2s ville orogeny. Apatite of Grenville age typically age uncertainties of either >25% or >100 m.y. exhibits high SREE (rare earth element), high were therefore excluded, similar to the proce- Th, and low Sr contents, and a pronounced negadure of Mark et al. (2016). tive Eu anomaly, all of which are indicative of an unmetamorphosed felsic plutonic source (e.g., RESULTS Belousova et al., 2002). The very low percentage One disadvantage of U-Pb detrital rutile and of Grenville-age rutile suggests that either there apatite dating is the relatively high proportion was minimal growth of rutile during the Grenof grains with low radiogenic (Pb*) to common ville orogeny, or that very low-grade metamor(Pbc) ratios. Because grains with low Pb*/Pbc are phic alteration of Grenville rutile occurred in the not uncommon, particularly for detrital apatite, WBR and the VR during the Phanerozoic (see disa high proportion of grains fail the age uncer- cussion). Phanerozoic-age apatite (in all samples) tainty rejection criteria, and are thus excluded is depleted in Th and the light REEs; this likely from the data set. In this study 11% of the rutile indicates growth synchronous with monazite or ages and 42% of the apatite ages were rejected. allanite during Appalachian metamorphism, and While other provenance studies may yield so the apatite trace element data further confirm detrital rutile and apatite with higher Pb*/Pbc, the greater significance of metamorphism versus it is recommended that in detrital apatite U-Pb plutonism during Appalachian orogenesis.
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DISCUSSION AND CONCLUSIONS The major apatite and rutile U-Pb age peaks largely do not correspond to the main monazite and especially the zircon U-Pb age peaks, which are dominated by Grenville zircon (Fig. 2). In contrast, the U-Pb apatite and rutile age data more fully characterize the full duration of Appalachian orogenesis, particularly the Neoacadian and Alleghanian tectonic phases (Fig. 2). The insensitivity of the U-Pb detrital zircon system to magma-poor Neoacadian and Alleghanian orogenesis in the southern Appalachians, a region central to the Paleozoic evolution of Laurentia, highlights the incompleteness of the detrital zircon record when investigating ancient orogenic systems; past orogenic episodes may be overlooked by narrowly focusing on the detrital zircon record. In addition, this study highlights the divergence in behavior between the apatite and rutile U-Pb systems, as the relative proportion of older apatite to rutile is greater in the detrital data set (Fig. 2). It is unexpected that the rutile data set primarily records the youngest (Alleghanian) event, while the apatite data set contains significantly more pre-Alleghanian detritus, despite the lower closure temperature of the U-Pb apatite system. This could be explained by rutile fertility differences between the various terranes, but is also likely a result of low-temperature breakdown of rutile to other Ti-bearing phases during retrogressive and low-grade metamorphic events (e.g., Triebold et al., 2012; Kohn et al., 2015). Low-grade alteration and breakdown of rutile results in the U-Pb rutile system being sensitive only to the most recent orogenic event in this segment of the Appalachian belt, as rutile from older events is consumed during each successive orogenic event. Due to the different U-Pb temperature sensitivities, crystallization histories, and fertility biases of different detrital chronometers, it would be unadvisable to assume that a single detrital phase could capture the full scope of past geological events in complex multiphase orogens such as the Appalachian or Alpine orogenic belts. Relying on a single detrital phase may misrepresent important geological events in certain tectonic settings, as demonstrated by the FBR detrital zircon record, which fails to detect the magma-poor Acadian and Alleghanian orogenic phases in this segment of the Appalachians. In comparison, we demonstrate that detrital apatite, in conjunction with detrital rutile and monazite, are far more faithful recorders of orogenesis in magma-poor collisional belts. Therefore, a routine multimineral approach to accessory mineral chronometry is strongly recommended to more fully characterize the detrital
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