Blueschist Facies Metaconglomerates: Catalina Schist, CA

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Altered dioritic clasts in lawsonite- blueschist facies metaconglomerates: Catalina Schist, CA

Natalie Elizabeth Sievers GEOL 394

Advisors: Dr. Sarah Penniston-Dorland Dr. William McDonough Dr. Philip Piccoli

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Table of Contents

Abstract ...... 2 I. Introduction ...... 3 II. Geologic Setting ...... 5 III. Objectives of Research ...... 8 IV. Discussion of Subduction Zone Chemistry ...... 8 V. Experimental Design and Approach ...... 9 VI. Results ...... 14 A. Petrographic Results ...... 14 B. Major Element Compositions ...... 16 C. Whole Rock Results ...... 17 D. Trace Element Mineral Results ...... 19

VII. Interpretations ...... 22 VIII. Acknowledgements ...... 23 Appendix A ...... 24 Appendix B ...... 28 Appendix C ...... 31 Appendix D ...... 33

References ...... 35

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

ABSTRACT The Catalina Schist, a Cretaceous subduction zone complex in California, displays an abundance of field, petrologic, and geochemical evidence for fluid-rock interaction during high P/T metamorphism. Metaconglomerates found in lawsonite-blueschist facies mélange are composed of gabbroic and dioritic clasts that display evidence of metasomatism under high P/T conditions (0.7-1.1 GPa, 300-400°C). Evidence for this metasomatism is visible at the outcrop scale in the form of quartz and calcite veins cutting across clasts, and at the petrographic scale with the appearance of phengite and lawsonite replacing igneous feldspars and sodic amphibole rimming igneous hornblende. Disseminated calcite in clasts and veins cutting across the clasts have a uniform oxygen isotopic composition that is similar to the oxygen isotopic composition of calcite in the surrounding mélange matrix and in veins in coherent metasedimentary and metamafic exposures, providing further evidence for large-scale fluid flow in the Catalina Schist.

In this study, the chemical changes due to metamorphism of these altered igneous clasts is described by comparing measured whole-rock major and trace element compositions of the lawsonite-blueschist metaconglomerate clasts to the composition of a possible source, the nearby Willows Plutonic Complex, and to arc diorites and gabbros from a number of localities worldwide. The clasts are variably enriched in LILE, including K2O, Rb, Ba, and Cs. These enrichments appear to correlate with the degree of observable petrographic replacements by high P/T minerals. In situ analysis of minerals using LA-ICP-MS demonstrates that phengite is the dominant host of the LILE, with up to 14 wt.% K2O, 7330 ppm Ba, 252 ppm Rb, 59 ppm Sr, and 12 ppm Cs. The other high P/T minerals have variable trace element compositions. Lawsonite has relatively high concentrations of Ba (up to 380 ppm) and Sr (up to 3494 ppm). The sodic amphibole has lower LREE and HFSE concentrations compared to the igneous hornblende it replaces. The oxygen isotopic composition implicates metasedimentary rocks as a potential source for the enrichments of LILE in the gabbroic and dioritic clasts.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

I. Introduction A. Subduction Zones and Fluids Subduction zones occur at convergent boundaries between two tectonic plates, where an oceanic plate is subducted under another tectonic plate, either oceanic or continental. As an oceanic plate subducts it carries a range of oceanic crustal lithologies to increasing depths and is exposed to changing pressures and temperatures. The subducted material varies spatially in composition by location and includes different proportions of unconsolidated and consolidated sediment, mid-ocean ridge basalt (MORB), altered basalt, and deeper sections of oceanic crust and associated lithospheric mantle. These rocks and sediments bring fluids, of varying compositions. Additionally, these rocks contain an abundance of hydrated minerals that can subsequently break down and release water over a wide range of pressures and temperatures, resulting in metasomatism in the slab (Manning, 2004). The fluids released during subduction rise into the overlying mantle wedge as depths and temperatures change within the subduction zone. Fluid flow in subduction zones plays a major role in tectonic processes, and plays a role in chemical changes in the subducting slab, the overlying lithospheric mantle, and any newly- generated continental crust. As the subducting plate increases in temperature and pressure, fluids are released. These fluids can ultimately rise into the mantle wedge, leading to arc volcanism and the generation of continental crust (Stern, 2002). Processes in both the subduction zone (subducting slab and mélange zone) and mantle wedge contribute to magma generation. Additionally, the movement of elements and the chemical compositions of volcanic rocks contribute to the generation and evolution of magma and the arc volcanic regions (Maurice et al, 2012). Arc-related igneous (volcanic and plutonic) rocks on the overlying plate reflect in part the composition of the material entering a subduction zone. These rocks have enrichments of elements (Ba, Sr, Rb, Cs, K, La, Th, and U) that have been shown to reflect inputs from the subducting slab. This pattern of enrichment provides evidence that some elements are mobile in subduction zones which leads to the enrichment of these mobile elements in the arc-related igneous rock (Plank and Langmuir, 1993).

During subduction the high field strength elements (HFSE; Zr, Nb, Ta, Th, Ti, and Hf) are considered to be relatively immobile relative to the large ion lithophile elements (LILE; K, Cs, Rb, Ba, Pb, Sr, plus Li, Na, and Ca) and the rare earth elements (REE) (Maurice et al, 2012; Spandler et al., 2003; Münker et al., 2004). This information can be used to evaluate the chemical contribution of the subducting slab to the mantle wedge by studying volcanic rocks and comparing them to metamorphosed rocks. By comparing these volcanic rocks to high P/T rocks, we can potentially provide evidence that these elements are mobile by observing trace elements redistribution, and enrichment or depletion in subduction-related metamorphic rocks. Additionally, by studying rocks that are interpreted to have been part of subduction zone assemblages, questions can be answered relating to the release and transport of subduction zone fluids. Some of these questions include determining: (1) the source of the fluid; (2) the chemical composition of the fluid; and, (3) the behavior of elements in this fluid as it is moving through a subduction zone (i.e. selectively entering or leaving mineral phases) (Manning, 2004).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

B. Mélange Zone Processes Exposed in subduction complexes there large domains that contain lithologic blocks that resemble the metamorphic rocks of the subduction complex. In addition to the presence of blocks, there mineral assemblages of hydrous minerals, and features such as rind development and cross cutting textures. Within this region there will be a mixture of rock packages varying in composition (mafic, ultramafic, and sedimentary) surrounded by a finer-grained matrix. These regions are thought represent the interface between the subducting slab and overlying mantle wedge. The blocks in the mélange zone reflect the incoming lithologies from the down- going slab and pieces of the overlying mantle wedge. As the slab subducts deeper fluid is released into the mélange zone and interacts with the lithological packages. The location of these mélange zones makes them ideal to record evidence of fluid dehydration and infiltration of fluids derived from the subducting slab. Fluids within a subduction zone must travel through the mélange zone to enter the mantle wedge, during which the fluid may pick up chemical signatures that are reflected in rocks generated in subduction zones (Penniston-Dorland et al., 2012). Due to this fluid transport, rocks in mélange zones are often highly enriched in fluid mobile trace elements. Fluid transport in mélange zones allows for elements that are usually incompatible to be mobilized and deposited in accessory minerals (Sorensen and Grossman, 1993). This element rich fluid infiltrates the overriding mantle wedge and contributes to the generation of arc magmas. Exhumed blocks from these mélange zones record this fluid and element mass transfer experienced during subduction (Breeding et al., 2004).

In addition to fluid-driven processes, regions of mixing at the interface between the down-going slab, composed of oceanic crust and sediment, and mantle wedge, can generate modified rock compositions, by mechanical mixing (Bebout and Barton, 2002; Penniston- Dorland et al., 2012). This process of mechanical mixing within subduction zones contributes to the transfer of elements and chemical signatures to rocks within the mélange zones and the chemical composition of the mélange matrix.

C. Lawsonite-Blueschist Metamorphism

Lawsonite-blueschist metamorphism occurs at temperatures and pressures of 300-400° C and 0.7-1.1 GPa, respectively, which corresponds to depths of about 40 km (Grove and Bebout, 1995) (Figure 1). Lawsonite-blueschist conditions correspond to the lower temperature limits of blueschist metamorphism. The temperatures experienced by these rocsk are much lower than the higher grade amphibolite rocks within the Catalina Schist which have peak conditions of 0.8 – 1.1 GPa and 640°C-750°C (Sorensen and Barton, 1987). The lower temperatures of lawsonite-blueschist metamorphism allow for the persistence of some igneous mineral phases and can result in a potentially diverse and extensive mineral assemblage for these rocks (Spandler et al., 2003). These minerals can include lawsonite ± sodic amphibole ± quartz ± white mica ± chlorite ± carbonate ± rutile ± titanite ± zircon ± relict igneous phases (hornblende, feldspar, apatite, and opaque phases) (Bebout et al., 2007; Tenore-Nortrup, 1995). Previous work has shown that during metamorphism volumes of fluid are released by dehydration reactions involving the various hydrous minerals (Clarke et al., 2006). During this fluid release there are also trace elements being released into the subducting slab. Therefore, by examining the trace element contents of hydrous metamorphic minerals in lawsonite-blueschist rocks within the Catalina Schist the elaborate fluid-rock interactions can be documented (e.g. Sorensen and Grossman, 1993; Spandler et al., 2003).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

The lawsonite-blueschist metaconglomerate clasts resemble unaltered igneous rocks but exhibit metasomatic alteration related with high P/T conditions. Metasomatic alteration of these igneous clasts during high P/T conditions offers opportunities to characterize the clasts in terms of the major, minor, and trace element compositions and to compare these findings to source material. By better defining the potential source material, and making comparisons to unaltered arc rocks from locations worldwide, using immobile trace elements, and then making observations about the compositions of the subduction-zone derived clasts, this study will provide insight to the detailed chemical Figure 1: Pressure-Temperature diagram showing changes that have occurred in these estimates for stability fields of sub-blueschist to rocks during subduction zone blueschist metamorphic facies (Bebout et al., 2007). processes. II. Geologic Setting A. Santa Catalina Island On Santa Catalina Island, California, the Catalina Schist consists of metamorphosed sedimentary and igneous rocks that were metamorphosed at sub-blueschist (lawsonite-albite) to amphibolite facies conditions (275°C - 750°C at 0.5 to 1.1 GPa) which correlates to subduction depths of 15 to 50km (Sorensen and Barton, 1987; Tenore-Nortrup, 1995; Grove and Bebout, 1995). These rocks are exposed over a wide area of Santa Catalina Island (Figure 3). Veining and alteration textures (such as metasomatic rind development in the amphibolite facies) are pervasive in the Catalina Schist, providing evidence that there has been extensive fluid-rock interaction at different pressure and temperature conditions within the subduction zone. Isotope studies also provide evidence of mixing during subduction zone metamorphism. Uniform δ15N values from white mica, and δ18O from carbonate material provides evidence of large-scale fluid flow during subduction zone metamorphism (Tenore-Nortrup, 1995; Bebout, 1997). These observations of large and small-scale fluid-rock interaction provide evidence that the chemical redistribution of elements could possibly be due to fluid-driven reactions.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

B. Willows Plutonic Complex

Exposed on Santa Cruz Island, California, there are three distinctive Jurassic rock units: the igneous Willows Plutonic Complex (WPC), and Alamos Pluton, and the metamorphic Santa Cruz Island Schist (Hill, 1976). Also exposed on the island are the metasediments of the Santa Monica Formation (Sorensen, 1985) (Figure 2). The rocks comprising the WPC include mafic (hornblende gabbros) and intermediate (hornblende diorites and hornblende-quartz diorites). Additionally, there is an area of saussurite gabbros that are interpreted as a product of alteration of gabbros and diorites (Hill, 1976). On small southern portion of Santa Catalina Island (Figure 3) there are also saussurite gabbros that chemically and texturally resemble those on Santa Cruz Island. For these reasons (lithologic assemblage, saussurite textures, proximity, and age), the WPC is considered to be a potential source region for the lawsonite- blueshist metaconglomerate clasts.

Figure 2: Geologic map of the southern coast of California, with Santa Cruz and Santa Catalina Island shaded in pink(after Sorensen, 1985).

C. Metaconglomerates There are three exposures of lawsonite-blueschist metaconglomerate blocks exposed on Santa Catalina Island, at Little Harbor, Shark Cove, and Lower Cottonwood Canyon (Tenore- Nortrup, 1995). The exposures at Little Harbor and Shark Cove are the focus for this study (Figure 3). Within the metaconglomerates, there is a wide range in lithologies, including metamorphosed sedimentary and igneous clasts. The relict igneous clasts range in composition

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

from felsic to mafic. These altered igneous clasts represent fragments of igneous rocks that were eroded, transferred and deposited on the sea floor and then entered a subduction zone as a conglomerate. The igneous lithologies include phaneritic intrusive clasts (mafic, intermediate), and aphanitic and phaneritic volcanic clasts (felsic). These different clasts are well rounded and unsorted within the metaconglomerate blocks themselves and range from centimeter to tens of centimeter in diameter. The metaconglomerate blocks are in contact with the finer-grained mélange matrix. The bulk chemical composition of the mélange matrix is a mixture of materials that originated from metasedimentary and mafic igneous lithologies in the mélange zone (Grove and Bebout, 1995). The mineral assemblage of the metaconglomerate matrix surrounding the clasts is similar to that of the lawsonite-blueschist facies blocks, but with the addition of talc, fuchsite, and stilpnomelane (Tenore-Norturp, 1995). These metaconglomerates, individual clasts, and matrix material show evidence for metasomatic interactions on the macroscopic and the microscopic scale.

Figure 3: Geologic map of Santa Catalina Island. The Catalina Schist is shaded in green and the two sample localities for this study are starred (After Grove and Bebout, 1995).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

III. Objectives of Research A. Objectives Samples for this study will include a suite of samples characterized by Tenore-Nortrup (1995), samples collected from the Catalina Schist in January of 2012, samples from the Willows Plutonic Complex (WPC), and published values for unaltered arc diorites. For this research, I propose to:

 make comparisons between the bulk rock major and trace element compositions of dioritic metaconglomerate clasts, the WPC, and arc diorites worldwide;  measure major and trace element compositions of relict igneous and metamorphic minerals in order to attribute enrichments of elements to the appearance of specific minerals;  determine the chemical changes and signatures left by the fluid interacting with these relict igneous clasts by making comparisons to the potential source region and arc regions worldwide;

B. Hypotheses

H0: There is no statistical difference in bulk-rock major and trace element chemical composition between unaltered arc diorites and the dioritic lawsonite-blueschist metaconglomerate clasts of the Catalina Schist

H1: Differences in major and trace element bulk-rock composition will be observed between the WPC source rocks, worldwide arc diorites, and sampled dioritic lawsonite- bueschist metaconglomerate dioritic clasts due to fluid-rock interactions. Particularly, I expect to see enrichments in the large ion lithophile elements (LILE: Cs, Rb, Ba, Pb, Sr), fluid mobile elements (FME: LILE plus Li), and rare earth elements (REE) relative to the high field strength elements (HFSE; Zr, Nb, Th, Hf, and Ta.

IV. Discussion of Subduction Zone Chemistry Seafloor sediments, sedimentary rocks, oceanic crust (including altered oceanic basalts), and altered oceanic basalts being subducted into convergent margins are thought to be enriched in the same suites of elements enriched in arc magmas. These magmas originate from the mantle wedge which is being enriched by fluids released from the underlying subducted package of rocks into the subduction zone (Münker et al., 2004). These elements are the LILE, and to some extent the LREE (Bebout 2007). Because these elements are fluid mobile the concentration of these elements can be measured to evaluate the concentration and contribution of the elements from the subducting slab to newly generated rocks. The addition of these elements from the subducting slab to the mantle wedge and then to volcanic rocks, and to newly forming metamorphic rocks can be studied to observe these additions or depletions. Past studies suggest that the fluid mobile elements, for example the LILE, are released into subduction zone fluids during mineral dehydration reactions (Hermann and Green, 2001; Spandler et al., 2003).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

The minerals that are breaking down in these reactions are often rich in trace elements. Within the lawsonite blueschist facies the primary hosts of these trace elements are lawsonite (REE, Rb, and Sr), titanite (REE, Nb, and Ta), and allanite (LREE, U, and Th) (Spandler et al., 2003). These redistribution processes trigger mineral reactions that are affected by changing conditions within the subduction zone. These dehydration reactions alter the mineral chemistry and modal element abundances of the rocks while leaving the mineral assemblage intact (Bebout et al., 2007). Previous studies on Santa Catalina Island rocks demonstrate that the LILE elements are being hosted primarily in white mica and chlorite, when present (Sorensen et al., 1997; Bebout et al., 2007).

The rare earth elements (REE) are also considered to be mobile during subduction zone metamorphism, less than the LILE have been shownto be, due to mineral dehydration reactions (Spandler et al., 2003). Lawsonite, titanite, allanite, epidote, and garnet are the primary hosts for REE for metamorphic rocks (Sorensen, 1991; Tribuzio et al., 1996; Spandler et al., 2003). Lawsonite and titanite will be the primary hosts for REE in the lawsonite-blueschist facies rocks. Studies have shown that lawsonite is usually enriched in the light (L) REE, Sr, and Pb. Titanite shows enrichments in the REE, Nb, and Ta, with slightly higher concentrations for the heavy (H) REE (Spandler et al., 2003). The process of adding or removing the bulk-rock REE concentration is dependent on various parameters within a subduction zone. Some of these parameters include; stability and solubility of minerals under high pressure/low temperature conditions, and the beginning mineral assemblages and proportions before entering a subduction zone (Tribuzio et al., 1996)

Zirconium, Nb, Ta, Th, Ti, and Hf are considered to have low mobility in subduction zone metasomatic processes when compared to the LILE or REE. These elements are typically immobile in subduction zone fluids, due to a high ratio of ionic charge to radius making it difficult to achieve charge balance with other ions (Frey, 2009), in the case of subduction zone processes water ions. These elements can be hosted in phases such as zircon, amphiboles, titanite, allanite (Spandler et al., 2003; Münker et al, 2004). The immobility of these elements is variable depending on the environmental conditions and these elements can be mobile with increasing temperature and pressure, or fluid composition, and evidence for mobility is seen in pegmatites and hydrothermal ore deposits (Jiang et al., 2005). Recent studies have shown that some of the HFSE can be mobile in subduction zone systems, with the constraint that multiple Ti-rich phases are present, and that high enough pressures are reached within the subduction zone (Münker et al, 2004). In order to normalize the data to observe which elements are being added or removed, the HFSE elements will be measured and compared to the presumed source rocks. Based on a ratio of mobile/immobile elements (i.e. Ba/Zr), it can be determined which elements are moving in and out of the rock and in what quantities. V. Experimental Design and Approach A. Previously Characterized Samples Previous studies (Tenore-Nortrup, 1995) analyzed approximately 60 thin sections of the lawsonite-blueschist rocks on Santa Catalina Island. Observations were made on intrusive clasts, metaconglomerate matrix material, and block and clast vein material. Whole rock major and trace element concentrations were determined using instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometer (ICP-MS) techniques.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Electron probe microanalyzer (EPMA) measurements were obtained for major element concentrations in minerals. Carbon, oxygen, and nitrogen isotope data was also collected for minerals and whole rock samples. Samples Collected As part of this study approximately 50 samples were collected at the Little Harbor and Shark Cove exposures in January 2012. The clasts collected range in igneous composition from felsic to mafic. The sample set was narrowed down based on available material for bulk rock analysis, petrography, and lack of weathered surfaces. B. GEOROC Database Samples To obtain a representative composition of global dioritic compositions, the GEOROC database was used to compile all available published data for plutonic rocks. This study is focused on making comparisons between the lawsonite-blueschist clasts and potential source rocks, for this reason values were selected from various arc regions at convergent margins. Once the region is selected, whole rock samples and major, minor, and trace elements were selected and downloaded. These data were then filtered on several criteria including, approroriate method of analysis, reasonable values for major elements (SiO2, and K2O), and trace elements (most notably anomalies in Ba, REE, and Zr concentrations). C. Bulk Rock Analyses Inductively Coupled Plasma Mass Spectrometer Solution (solution ICP-MS) Approximately 20 mg of bulk rock powder was digested in a 3:1 mixture of concentrated distilled HF and HNO3 acids in Teflon Parr bombs at >160 °C for 72 hours along with standards BHVO 1 and B (Appendix D) and an analytical blank. Teflon digestion bombs were cooled, and solutions were transferred into clean Teflon beakers. Residues were repeatedly taken up and dried down in HNO3 and HCl acid. Residual solution was taken up in 1ml of HNO3 acid, diluted and spiked with Indium to account for drift and normalization during data processing. Rock dissolutions were completed in the Clean Laboratory and ICP- MS analyses were done in the Plasma Mass Spectrometry facilities using a Thermo-Finnigan Element 2 ICP-MS, all at the University of Maryland. Trace elements measured include Sc, Ti, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Th, Ta, Pb, and U. X-ray Fluorescence (XRF) Major and minor elements were analyzed using an x-ray fluorescence vacuum spectrometer equipped with a 4kW Rh anode super sharp X-ray tube by Dr. Stanley Mertzman in the X-ray laboratory at Franklin and Marshall College. Element and oxides reported include: SiO2, Ti02, Al203, Fe203, MnO, MgO, CaO, Na20, P2O5 and K20, LOI (loss on ignition), and Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Nd and Ga.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

D. Mineral Analyses

Petrographic Microscope Five minerals of interest were identified for analysis: phengite, sodic amphibole, lawsonite, titanite, and relict igneous phases of hornblende. Additionally, textural relationships among minerals, such as the presence of pseudomorphs, and lawsonite clustering, were documented. Based on these textures, mineral grains were chosen for analyses. Accessory phases were also identified to complete mineral assemblages. Grains for analyses were chosen based on (1) size, (2) degree of alteration (3) absence of inclusions and cracks and, (4) distinguishable from surrounding minerals and textures for later EPMA and LA-ICP-MS methods. The petrographic microscope was used to make maps for use during EPMA and LA- ICP-MS techniques. Map making requires taking photographs in transmitted light (plane polarized and cross polarized light [PPL and XPL, respectively]), and reflected light. Electron Probe Microanalyzer (EPMA) Major and minor element (K, Ca, Na, Ba, Al, Mg, Fe, Si, and Ti) concentrations in minerals were measured using the JEOL JXA-8900 electron probe micro analyzer (EPMA) at the University of Maryland. Areas for analysis were chosen based on; size, homogeneity of sample, and absence of inclusions. A 200-300 Å thick layer of carbon was used to cover all samples before analysis to act as a conductor. The phases were then analyzed by using a beam potential of 15kV, with a beam diameter of 1- 10 μm and a 10-20 nA cup current. Energy dispersive spectrometry (EDS) was often used initially to confirm previous phase identification from petrographic analysis. This was followed by wavelength dispersive spectrometry (WDS) to collect major and minor element concentrations. These concentrations were then compared to standard reference material (see Appendix D) for the appropriate mineral phases as a test of measurement accuracy. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) Trace element and selected major-element data were collected using a NewWave frequency-quintupled Nd-YAG (Y-Al-Garnet) laser (213 nm) coupled to a Thermo-Finnigan Element 2 ICP-MS at the University of Maryland from thin sections. The sample after ablation was carried via He gas and entered the plasma. Background signal was collected for approximately 20 seconds and sample signal was collected for approximately 50 seconds Analysis spot size ranged depending on the phase from 40μm to 55μm. Standard reference materials used in these LA-ICP-MS techniques were, NIST610, BIR-1G, BHVO-2G (see Appendix D Table 1 for values). These glass standards are analyzed before and after the analyses of minerals in this study. Concentrations of the internal standards, Ca or Si, from EPMA methods were used for normalization. The software program LAMTRACE was used to process the data. Elements analyzed include the elements considered fluid mobile during subduction zone processes: the large ion lithophile elements (LILE): Cs, Rb, Ba, Pb, Sr, plus Li; the high field strength elements (HFSE): Zr, Hf, Nb, Th, and Ta; and the rare earth elements (REE).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

E. Error Analysis The uncertainty associated with the EPMA analysis is calculated using counting statistics, 1 √ , where n is the number of counts collected for a specific oxide during an analysis. For ICP-MS methods USGS standards were analyzed concurrently with samples. From the data collected the relative standard deviation, 100 , was then calculated to determine the precision of the analyses. VI. Results of Previous Studies A. Field Observations Both Little Harbor and Shark Cove (Figure 2) exposures of metaconglomerate blocks display similar evidence for extensive fluid-rock interaction. Large veins of quartz and calcite (Tenore-Nortrup, 1995) cross-cut the metaconglomerate blocks as well as individual clasts. Additionally there is quartz and calcite occurring throughout the metaconglomerate clasts themselves. Slight mineralogical and/or compositional variations from the clast exteriors inward are also observed suggesting that the perimeters of the clasts were altered more than the interior by fluids. These textures can also be seen petrographically. Within the metaconglomerate matrix there are large pods of hydrous phases providing evidence of pervasive fluid interaction.

D A

C B

12 Figure 4: Images A-C are large metaconglomerate blocks exposed at field sites. Image D shows thin section SC12-I4A, metaconglomerate matrix on left and clast interior on right. Euhedral feldspar phenocrysts replaced by phengite. Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

B. Geochemical Observations

Previous studies have measured δ18O, δ13C, and δ15N of the clasts from the lawsonite- blueschist metaconglomerate blocks (Tenore-Nortup, 1995). Carbonate from within clasts, veins Awithin clasts, veins around clasts, veins within the metaconglomerate matrix, and carbonate dispersed within the matrix all were analyzed for δ18O and δ13C. The δ18O values are uniform (+14.6 ± 0.4‰) between the different clasts in the metaconglomerates and each of the individual metaconglomerate outcrops (Tenore-Nortrup, 1995). Prior to entering a subduction zone and undergoing metamorphism, these rocks would have had different isotopic compositions; the uniformity among the isotopic composition suggests that these rocks have all interacted with a fluid of the same composition (Figure 5).Bulk rock δ15N isotope analysis was also completed on the rocks from the lawsonite- blueschist metaconglomerate clasts (Bebout and Fogel, 1992; Bebout, 1995; Bebout, 1997; Tenore-Nortup, 1995). Nitrogen in these rocks is + hosted primarily by white mica as NH4 , but can also reside in plagioclase. The amount of unaltered plagioclase remaining in these rocks is very low so the δ15N isotope data reflects the values for micas. Unmetasomatized igneous rocks typically contain small amounts of N (1-5 ppm),B and because the data from these previous studies Cshows enrichments of nitrogen well above those low concentrations (up to 730 ppm), it has been inferred that this increase in nitrogen is due to fluid transfer and devolatization reactions during subduction zone metamorphism (Tenore-Nortrup, 1995). Likewise, the nitrogen isotopic compositions from different minerals and veins within the separate outcrops are uniform (mean value +2.4‰) suggesting that the fluid interacting with the rocks was similar on the large scale (Tenore- Nortrup, 1995) (Figure 5).

Figure 5: Uniformity of δ15N (left) and δ18O (right) isotope values suggests uniformity of fluid compositions for the fluid moving through these metaconglomerate blocks (Tenore-Nortrup, 1995).

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

VII. Results A. Petrographic Observations Catalina Schist Lawsonite-Blueschist Clasts Using the petrographic microscope petrographic mineral assemblages, abundances and textures were observed. Textures include cross cutting veins, rimming of sodic amphibole around igneous hornblende (Figure 6), phengite after feldspar, and the addition of hydrous phases such as lawsonite (Figure 7). Amphiboles Amphiboles within the metaconglomerate clasts occur as elongate, fibrous, felted, or acicular, anhedral to subhedral grains. Most grains show rimming textures of relict amphibole cores with Na-rich amphibole rims. The size of the grains is variable between the clasts, ranging from .2mm to 3cm. Dark green-brown to light green relict hornblende amphiboles are found most often as the cores of the amphibole growth generations as anhedral to euhedral cores, ranging in size from 4mm to 1cm. Compositionally the phases range from magnesiohornblende (Mg rich calcic amphibole) to actinolite (Mg – Fe rich calcic amphibole) (Appendix A).Sodic amphiboles occur as blue-violet growths around the relict igneous amphiboles. Often there is a gradation color change in the fibrous amphibole rims changing from dark violet to pale purple- blue. Sizes of these growths range from 2mm up to 5mm. Single fibrous blades of sodic amphibole can be seen scattered throughout a sample and sometimes found within phengite grains (Figure 7).

Figure 6 (left) and 7 (right): Sample 9-1-50d in PPL. Figure 5 illustrates the rimming of the new Na-rich amphibole around a relict igneous amphibole. Figure 6 shows the replacement reaction of feldspar to phengite (outlined in purple) and small lawsonite occurring as large masses around these grains (orange). Figure 6 also shows single blades of Na-rich amphibole occurring inside phengite gains.

Lawsonite (CaAl2Si2O7(OH)2 • H2O Lawsonite grains are present in most samples occurring mostly as small, (1mm to 2mm) euhedral grains occurring in clusters. Rarely lawsonite occurs as large prismatic grains with sizes up to 1cm. Texturally, the grains are found most often as matted growths rimming

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

phengite psuedomorphs Phengite rimming of igneous feldspar is variable between different clasts. Phengite (Mg-Si rich muscovite) Phengite is present in all samples as fine grained replacements or disseminated patches throughout a sample. Most notably phengite will totally replace the igneous feldspar phase but retain the euhedral habit and polysynthetic twinning characteristic of feldspars (Figure 8).

Figure 8: Photomicrograph of SC03-02 half in PPL/XPL showing the residual feldspar polysynthetic twinning.

Titanite (CaTiSiO5) Titanite, when present, occur as small euhedral grains ranging in size from 2mm to 5mm. Very often titanite is associated with rutile, magnetite, or ilmenite.

Accessory Phases

Accessory phases include magnetite, epidote, rutile, apatite, chlorite, zircon, and carbonates.

Willows Plutonic Complex

Samples from the WPC are primarily fine grained with large amphibole phenocrysts abundant in thin section (Figure 9). Other phases such as feldspar are present, but are too small to perform EPMA or LA-ICP-MS analyses.

15 Figure 9: Photomicrograph of WPC sample 827808 in PPL showing large amphibole phenocryst. Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

A. Major Element Compositions

Seven samples (Figure 10) were analyzed using the EPMA to determine the major- element mineral compositons of minerals from the WPC and lawsonite-blueschist clasts. The minerals analyzed include amphibole, phengite, lawsonite, and titanite. The average relict igneous hornblende composition is magnesiohornblende and actinolite. This contrasts the sodic amphibole rims where Na replaces most of the Ca, and is compositionally winchite (Figure 11) (Hawthorne et al., 2007). The amphiboles analyzed for WPC samples are actinolite- magnesiohornblende variety, while the amphiboles in the lawsonite-blueschist clasts are mostly magnesiohornblende, with one clast having amphiboles with more actinolitic compositions.

Figure 10: Table of samples and different phases that were analyzed. 827808 – diorite from WPC.

Figure 11: Average mineral compositions for six lawsonite-blueschist clasts and one WPC sample (827808), using MSA classification scheme.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

B. Whole Rock Results To evaluate: (a) if the Willows Plutonic Complex (WPC) are the source rocks for the Catalina Schist, (b) how the lawsonite-blueschist rocks compare to global dioritic compositions, and (c) what elements may have been added to or removed from rocks during metamorphism, the data collected from the clasts was compared to collected values for the WPC and published values for global arc volcanoes to monitor changes in the fluid mobile elements. Major element comparisons between the lawsonite-blueschist dioritic metaconglomerates, WPC, and published values for arc diorites, show that the lawsonite-blueschist clasts are enriched in K2O, MgO and TiO2 (Figure 12)(see Appendix A for values). Potassium, an element considered mobile during subduction zone processes, is close to four times enriched relative to the WPC (Figure 13).

Figure 12: Comparison of major-element compositions of lawsonite-blueschist clasts, WPC diorites, and GEOROC diorite compositions. Solid lines represent the average, and fields represent the range of values for a given region. Pink – lawsonite-blueschist metaconglomerate clasts, blue – GEOROC range of values, green – WPC.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Figure 13: Comparison of major-element compositions of lawsonite-blueschist clasts to WPC diorites and to GEOROC diorite compositions.

Figure 14: Comparison of trace element compositions of lawsonite-blueschist clasts to WPC diorites, 18 and lawsonite-blueschist clasts compared to GEOROC diorite compositions. Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Figure 15: Comparison of trace element compositions of lawsonite-blueschist clasts to WPC diorites and to GEOROC diorite compositions.

C. Trace Element Mineral Results Trace elements collected from minerals also show enrichments of LILE (Cs, Rb, Ba, Pb, Sr) relative to pyrolite. Calcium rich amphiboles have notable enrichments in REEs, with small concentrations of some of the HFSE (Zr, Nb, and Ta). The Na-rich amphiboles have note- worthy concentrations of some of the fluid mobile elements (Rb, Sr, Cs, and Ba) (see Appendix B for values). Phengite is highly enriched in the LILE, especially Rb, Cs, and Ba. This is also the case for lawsonite, which in addition to LILEs is hosting REEs. Titanite has moderate enrichments of Ba, but more interesting, titanite hosts many HFSE, Zr, Nb, Th, Ta, and U.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Figure 16: Trace element concentrations of amphiboles. Green line represents the average composition of all amphiboles in WPC sample 827808. Pink and blue line represents the average composition of Ca- rich and Na-rich amphiboles, respectively, in lawsonite-blueschist clasts.

Figure 17: Trace element concentrations of phengite in lawsonite-blueschist clasts. Line represents the average concentrations in all samples analyzed.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Figure 18: Trace element concentrations of titanite in lawsonite-blueschist clasts. Line represents the average concentrations in all samples analyzed.

Figure 19: Trace element concentrations of lawsonite in lawsonite-blueschist clasts. Line represents the average concentrations in all samples analyzed.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

XII. Interpretations The enrichments of fluid mobile elements in the lawsonite-blueshist clasts, and the appearance of metamorphic facies index minerals suggests a possible alteration by fluids. The lawsonite-blueschist clasts are enriched in K2O, Rb, Sr, Ba, and Pb relative to the WPC and enriched in Sr, Ba, Pb relative to global dioritic compositions. The lawsonite-blueschist clasts are over 3 times more enriched in K2O. The lawsonite-blueschist clasts are over 17 times and 10 times enriched in Ba and Rb, respectively, when compared to the WPC. The lawsonite-blueschist clasts are depleted for the REE measured (La and Ce) for both the WPC and global dioritic compositions, with the exception of a slight enrichment of Ce in the clasts relative to the WPC.The HFSE are considered fluid immobile in subduction zone systems, thus if the WPC is a source region for the lawsonite-blueschist clasts the HFSE analyzed (Ti, Zr, Nb, Hf, Ta, Th, and U) should have a 1:1 ratio. The ratio of the WPC to the lawsonite-blueschists clasts is close to 1, and this is also the case for global dioritic concentrations of HFSE when compared to the metaconglomerate clasts. Given that the GEOROC and WPC sample set will be heterogeneous, these values likely fall within a range of values that are considered acceptable for defining if two populations of samples are accepted as chemically different. The sample set available at this time for the WPC is limited so an appropriate method for determining a range of values that the lawsonite-blueschist compositions would have to fall within in order to be considered similar is unavailable. However, there are cases such as K2O, where the values for the clasts would likely fall outside of this range of values for heterogeneous dioritic clasts. Thus, K is likely enriched due to fluid- rock interactions during subduction. Mineral analyses show which phases are contributing a given element to the overall rock budget. Phengite is enriched relative to pyrolite in many of the LILE (Rb, Sr, Cs, and Ba) (Figure 17). Lawsonite is very concentrated in these elements with up to 282 ppm Rb, 272 ppm Sr, up to 19.8 ppm Cs and 6925 ppm Ba. Lawsonite is also enriched in the same LILE as well as REEs. Titanite hosts much of the HFSE with concentrations of484 ppm Zr, 582 ppm Nd, and 29 ppm Ta, as well as 42000 ppm of Ti. Each population of amphibole (WPC Ca-rich, and the lawsonite blueschist Ca-rich and Na-rich) have surprisingly different trace element concentrations. All are enriched relative to pyrolite for some LILE (Rb, Sr, Ca, and Ba). In terms of the REE, the WPC amphiboles show enrichments of the REE with a general decrease in concentration with increasing mass. The Na-rich amphiboles are depleted relative to pyrolite in all REEs which strongly constrasts the trace element concentrations of the Ca-rich amphiboles in clasts. Willows Plutonic Complex amphiboles have low concentrations of HFSE relative to pyrolite, with ratios close to one. This strongly contrasts both amphibole varieties in lawsonite-blueschist clasts. Sodium-rich amphiboles from clasts are depleted in Ti, Zr, Nb, Th, Ta , and U with tantalum showing the largest depletion of almost 10 times depleted. Hafnium is the only HFSE element that is enriched. Calcium-rich amphiboles from the dioritic lawsonite-blueschist clasts are enriched when compared to pyrolite in most of the HFSE including, Ti, Zr, Nb, Hf, Ta, and U. With these amphiboles Ta is almost 10 times enriched rather than depleted. The one exception to the HFSE enrichment is Th, which is slightly depleted relative to pyrolite.

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

The presence of hydrous metamorphic index minerals (sodic amphibole, lawsonite, and phengite) provides evidence that these rocks were altered by fluids but the enrichment of these fluid mobile elements in these minerals offers more evidence that these rocks are being altered by fluid. Furthermore, the enrichments seen in the bulk rock data can be attributed to the apperance of new minerals and the enrichment of specific elements in these minerals (i.e. phengite can be attributed to the enrichment of K2O, Rb, Ba, and Cs). VIII. Acknowledgements: I would like to thank my wonderful advisors who have provided me with unwavering support and guidance over the course of the project. Dr. Gray Bebout at Lehigh University, Dr. Walker, and Julia Gorman for their assistance in the field and direction as my project was developing. Thanks to Dr. Candela, Dr. Kaufman, and Dana Borg for their many edits or general support over the course of the project. Special thanks to Dr. Richard Ash for helping with anything dealing with the mass specs!

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Appendix A:1 Major-element compositions of amphiboles

23 Table 1: Measured values for major-element compositions of amphiboles. “a” samples indicate relict Ca-rich amphiboles, “b” samples represent Na-rich amphiboles. Appendix A:2 Major-element compositions of phengite

Table 1: Measured values for major-element compositions of phengites. “a” and “b” indicates a single grain was analyzed multiple times. * represents a samples whose total weight fell outside the accepted values for mineral stoichiometry.

Appendix A:3 Major-element compositions of lawsonite

Appendix A:4 Major-element compositions of titanite

Altered dioritic clasts in metaconglomerates: Catalina Schist, CA

Appendix B:1 Trace-element compositions of amphiboles

Table 1: Measured values for trace-element compositions of amphiboles. * indicates when the internal standard, Si, was calculated using the average of past EPMA results.

Appendix B:2 Trace-element compositions of phengite

Appendix B:3 Trace-element compositions of lawsonite

Appendix B:4 Trace-element compositions of titanite

Appendix C:1 Major-element compositions of lawsonite-blueschist clasts and WPC

Appendix C:2 Major-element compositions of GEOROC dataset

Appendix D:

Table 1: Glass reference standards used in EPMA methods.

Table 2: USGS glass reference standards used in LA-ICP-MS methods and values measured from standards in this study. n.r = not reported. SiO2-TiO2 given as weight percent. Sc-Lu in ppm. 33

Table 3 and 4: Reference standard values used in solution-ICP-MS methods and values measured from standards in this study. n.r = not reported. SiO2-TiO2 given as weight percent. Sc-Lu in ppm. Pyrolite reference values from McDonough and Sun, 1995.

University of Maryland Honor Pledge

I pledge on my honor that I have not given or received any unauthorized assistance or plagiarized on this assignment.

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