Graphite‐schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high‐P metamorphism
Graphite‐schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high‐P metamorphism
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加利福尼亚州圣西蒙方济会混杂岩中的石墨片岩块：高磷变质作用的证据
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发表时间：
2016
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通讯作者：
M. Cloos
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作者：
E. Ukar;M. Cloos
加利福尼亚州圣西米恩的Seacliff发现，页岩基质中含有石墨片岩区块，这是方济各会混杂岩中一种未被记录的岩性。对30块石墨片岩块进行了研究，以发现这一经典混杂地区的所有品种。根据矿物组合、成分和结构特征，圣西缅岛的石墨片岩可分为两种主要类型(类型I和类型II)，每种类型又分为两个亚类(A和B)。IA型和IIA型区块最为丰富。IA型石墨片岩为粉砂岩/细灰砂岩，保存有沉积结构，不含钠铝石。IB型石墨片岩在矿物学和结构上与IA型片岩相似，但颗粒更细的粉砂岩和页岩。IIA型石墨片岩在成分上呈层状，含有富石英和钠长石的层，以及深色的云母/绿泥石层。15个IIA型区块中有9个含有劳钠铝石。两个IIA型区块的矿脉中也含有文石(+方解石)。IIB型石墨片岩主要由石英和少量石墨组成，绿泥石和白云母互生，白云母假象类似于劳钠铝石。IIA型块体中云母的多硅白云母含量高于IA型石墨片岩中的云母含量，证实它们是在高P/低T条件下变质的。IA型块体在200~250℃时重结晶，而IIA型块体在更高的压力条件下变质，可能在250~300℃和3~5kbar。这两种类型的石墨片岩最有可能来自相似的层状粉砂岩/细灰砂岩/页岩原岩。在Franciscan俯冲过程中，海沟轴线上沉积的富含有机质的沉积物与洋壳一起被俯冲。I型石墨片岩被俯冲至10千米深处，而II型石墨片岩被俯冲至约15千米深度，在那里它们在高磷条件下被底侵。在俯冲通道剪切带中，石墨质变质沉积岩与俯冲洋壳的镁铁质岩性并置，变质为蓝片岩相，并退变为绿岩。
The seacliff exposure at San Simeon, California, contains graphite‐schist blocks in a shale‐matrix, an undocumented lithology within the Franciscan mélange. Thirty graphite‐schist blocks were studied to discover all the varieties in this classic locality of mélange. Based on their mineralogical assemblage and composition, and textural characteristics the graphite‐schists in San Simeon are subdivided into two main types (Type I and II) with two subdivisions each (A and B). Type IA and IIA blocks are the most abundant. Type IA graphite‐schists are siltstone/fine greywacke‐like, preserve sedimentary textures, and lack lawsonite. Type IB graphite‐schists are mineralogical and texturally similar to Type IA schists, but are finer grained siltstone and shale. Type IIA graphite‐schists are compositionally layered and contain quartz‐ and albite‐rich layers and dark graphite‐ and intergrown mica/chlorite‐rich layers. Nine out of the 15 Type IIA blocks contain lawsonite. Two Type IIA blocks also contain aragonite (+calcite) in veins. Type IIB graphite‐schists are mostly composed of quartz and minor graphite, intergrown chlorite and white mica, and white mica pseudomorphs after lawsonite. The phengite content of mica in Type IIA blocks is higher than that of mica in Type IA graphite‐schists, confirming they were metamorphosed under high‐P/low‐T conditions. Type IA blocks were recrystallized between 200 and 250 °C at <~3 kbar; whereas, Type IIA blocks were metamorphosed under higher pressure conditions, probably at 250–300 °C and 3–5 kbar. Most likely both types of graphite‐schists were derived from a similar layered siltstone/fine greywacke/shale protolith. Organic matter‐rich sediments deposited in the trench axis were subducted along with oceanic crust during Franciscan subduction. Type I graphite‐schists were subducted to depths <10 km, whereas Type II graphite‐schists were subducted to depths ~15 km where they were underplated under high‐P conditions. The graphitic metasedimentary rocks were juxtaposed with mafic lithologies from the subducted oceanic crust that were metamorphosed to blueschist facies and retrograded to greenstone as they returned to the surface in the subduction channel shear zone.