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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 0 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 1 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. 2 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). 3 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.
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