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2014
Chin, EJ, Lee C-TA, Barnes JD.  2014.  Thickening, refertilization, and the deep lithosphere filter in continental arcs: Constraints from major and trace elements and oxygen isotopes. Earth and Planetary Science Letters. 397:184-200.   10.1016/j.epsl.2014.04.022   AbstractWebsite

Arc magmatism is a complex process involving generation of primary melts in the mantle wedge and chemical refinement of these melts into differentiated products akin to continental crust. Interaction of magmas (cooling, crystallization and assimilation) with the overlying crust, particularly if it is thick, is one way by which primary basalts are refined into more evolved compositions. Here, we explore the role of the mantle lithosphere as a trap and/or reactive filter of magmas. We use mantle xenoliths from the Sierra Nevada continental arc in California as a probe into sub-Moho processes. Based on clinopyroxene modal abundance and major, minor and moderately incompatible trace element concentrations, the peridotites define a refertilization trend that increases with depth, grading from clinopyroxene-poor ( < 5 % ), undeformed spinel peridotites equilibrated at < 3 GPa ( < 90 km ) to clinopyroxene-rich (10–20%), porphyroclastic garnet peridotites equilibrated between 3 and 3.5 GPa (90–105 km), the latter presumably approaching the top of the subducting slab. The petrology and geochemistry of the xenoliths suggest that the fertile peridotites were originally depleted spinel peridotites, which were subsequently refertilized. Incompatible trace element geochemistry reveals a pervasive cryptic metasomatic overprint in all peridotites, suggesting involvement of small amounts of subduction-derived fluids from the long-lived Farallon plate beneath western North America. However, bulk reconstructed δ O SMOW 18 values of the peridotites, including the most refertilized, fall between 5.4 and 5.9 ‰ , within the natural variability of unmetasomatized mantle ( ∼ 5.5 ± 0.2 ‰ ). Together with Sm, Yb, and Ca compositional data, the oxygen isotope data suggest that the role of slab or sediment melts in refertilizing the peridotites was negligible ( < 5 % in terms of added melt mass). Instead, binary mixing models suggest that many of the Sierran garnet peridotites, particularly those with high clinopyroxene modes, had up to 30% mantle-derived melt added. Our data suggest that refertilization of the deep arc lithosphere, via melt entrapment and clinopyroxene precipitation, may be an important process that modifies the composition of primary arc magmas before they reach the crust and shallowly differentiate. Comparison of our data with a global compilation of arc-related mantle xenoliths suggests that sub-Moho refertilization may be more extensive beneath mature arcs, such as continental arcs, compared to juvenile island arcs, possibly because of the greater thickness of crust and lithosphere beneath mature and island arcs.

2013
Chin, EJ, Lee CTA, Tollstrup DL, Xie LW, Wimpenny JB, Yin QZ.  2013.  On the origin of hot metasedimentary quartzites in the lower crust of continental arcs. Earth and Planetary Science Letters. 361:120-133.   10.1016/j.epsl.2012.11.031   AbstractWebsite

Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U-Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U-Pb ages (103 +/- 10 Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700-800 degrees C using Ti-in-quartz thermometry and 0.7-1.1 GPa using garnet-aluminosilicate-plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible partial melting in the lower crust, most likely related to arc magmatism as suggested by similarities between the lower intercept U-Pb ages and the ages of plutonism in the Sierra Nevada. Possible mechanisms by which these sediments were transported into the lower crust include continental underthrusting beneath the continental arc, underplating by buoyant slab-derived sedimentary diapirs, or viscous downflow of country rock in response to diapiric ascent of plutons. Continental underthrusting has been independently documented during the Sevier orogeny, coinciding with the peak of arc magmatism. We thus speculate that supracrustal rocks may have been underthrusted into deep crustal magmatic zones. Regardless of how these metasediments arrived in the lower crust, our observations indicate that silicic metasediments occur in the lower crust of volcanic arcs, not just in the upper crust as is commonly thought. Transport of metasediments into deep crustal magmatic zones should influence the composition of arc magmas and continental crust in general. (C) 2012 Elsevier B.V. All rights reserved.

2012
Chin, EJ, Lee CTA, Luffi P, Tice M.  2012.  Deep Lithospheric Thickening and Refertilization beneath Continental Arcs: Case Study of the P, T and Compositional Evolution of Peridotite Xenoliths from the Sierra Nevada, California. Journal of Petrology. 53:477-511.   10.1093/petrology/egr069   AbstractWebsite

Thickening of arc lithosphere influences the extent of magmatic differentiation and is thereby important for the evolution of juvenile arcs into mature continental crust. Here, we use mantle xenoliths from the late Mesozoic Sierra Nevada continental arc in California (USA) to constrain the pressure, temperature, and compositional evolution of the deep lithosphere beneath a mature arc. These xenoliths consist of spinel peridotites and garnet-bearing spinel peridotites. The former are characterized by coarse-grained protogranular textures having bulk compositions indicative of high-degree melting. The latter are characterized by porphyroclastic textures, garnet coronas around spinels, garnet exsolution lamellae in pyroxenes, and pyroxenes with high-Al cores and low-Al rims. The garnet-bearing spinel peridotites range from depleted to fertile compositions, but the high Cr-numbers [molar Cr/(Cr + Al)] of spinel cores reflect high-degree melting. These observations suggest that the protoliths of the garnet-bearing spinel peridotites were melt-depleted spinel peridotites. Constraints from geothermobarometry and bulk compositions coupled with mantle melting models suggest that the protoliths underwent shallow melt depletion (1-2 GPa, 1300-1400 degrees C), followed by compression, cooling, and final equilibration within the garnet stability field (similar to 3 GPa, < 800 degrees C). The deepest equilibrated samples are the most refertilized, suggesting that refertilization occurred during compression. We interpret this P-T-composition path to reflect progressive thickening of the Sierran arc lithosphere perhaps as a result of magmatic inflation or tectonic thickening. We hypothesize that newly formed arc lithospheric mantle thickens enough to pinch out the asthenospheric wedge, juxtaposing Sierran arc lithosphere against the subducting oceanic plate. This could have terminated arc magmatism and initiated cooling of the deep Sierran lithosphere.