Field-Petrochemical Investigation of Ultrahigh-Pressure Garnet Periodotites from the Sulu Terrane, Eastern China

以前的研究允许在苏鲁-大别超高压(UHP)地体中识别出两种不同类型的石榴石橄榄岩。A型橄榄岩代表中朝板块地幔楔体的碎片，B型橄榄岩是俯冲前侵位于陆壳的扬子板块的壳源镁铁质-超镁铁质杂岩。这些橄榄岩中的橄榄石、石榴石和辉石含有微小的矿物包裹体，它们可能是非常高的P相或致密的含水镁硅酸盐。此外，石榴石、绿辉石和多硅白云母记录的最负的三角洲18OO值表明，在超高压变质作用和随后的折返过程中，流体/岩石相互作用极其有限；因此，原始地幔特征有望得到很好的保存。这项研究架起了地幔岩石学和矿物物理学之间的桥梁，并将解决以下问题：(1)超高压石榴石橄榄岩能否为上地幔的岩石构造过程提供新的约束？(2)矿物的地球化学微观分析对了解地幔-板片和岩石/流体相互作用过程有何贡献？(3)超高压矿物中出溶片层的意义是什么？(4)还有什么矿物学证据记录了石榴石橄榄岩的超高压变质作用？苏鲁地体岩芯和地表样品的主要逆冲断层包括：(1)石榴石橄榄岩的岩石学特征和变质演化；(2)其来源和初始构造环境的地球化学制约；(3)放射性和稳定同位素组成，以评价壳/地幔和岩石/流体的相互作用；(4)各种方法确定年龄，包括锆石的SHRIMP定年。这种综合的岩石化学和地质年代学约束是评估与大陆碰撞和俯冲有关的上地幔性质的先决条件。

9814468LiouPrevious studies allowed identification of two distinct types of garnet peridotites in the Sulu-Dabie ultra-high pressure (UHP) terrane. Type A peridotites represent fragments of the mantle wedge of the Sino-Korean plate and Type B peridotites are crustal-derived mafic-ultramafic complexes of the Yangtze plate emplaced into continental crust prior to subduction. Olivine, garnet, and pyroxene of these peridotites contain minute mineral inclusions that may be very high-P phases or dense hydrous magnesian silicates. Furthermore, the most negative delta 18-oxygen values ever recorded for garnet, omphacite and phengite suggests extremely limited fluid/rock interactions during UHP metamorphism and subsequent exhumation; thus primary mantle features are expected to be well preserved. This research bridges the gap between mantle petrology and mineral physics and will address the following questions: (1) Can UHP garnet peridotites provide new constraints on petrotectonic processes of the upper mantle? (2) What can geochemical microanalyses of minerals contribute to understanding the processes of mantle-slab and rock/fluid interactions? (3) What is the significance of exsolution lamellae in UHP minerals? (4) What other mineralogical evidence records the UHP metamorphism of garnet peridotites? Major thrusts on both core and surface samples from the Sulu terrane include: (1) petrological characteristics and metamorphic evolution of garnet peridotites; (2) geochemical constraints for their source and initial tectonic setting; (3) both radioactive and stable isotopic compositions to evaluate interactions of crust/mantle and rock/fluid; and (4) age determinations by various methods including SHRIMP dating of zircons. Such comprehensive petrochemical and geochronological constraints are prerequisites to evaluate the nature of the upper mantle associated with continental collision and subduction.