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

[9814468]以往的研究在苏鲁-大别超高压（UHP）地层中发现了两种不同类型的石榴石橄榄岩。A型橄榄岩为中朝板块地幔楔块碎片，B型橄榄岩为扬子板块俯冲前侵位大陆地壳的基性-超基性杂岩。橄榄石、石榴石和辉石的这些橄榄岩含有微小的矿物包裹体，可能是非常高的p相或致密的含水镁硅酸盐。此外，石榴石、辉长石和辉长石的δ 18氧值为负，这表明在超高压变质和随后的挖掘过程中，流体/岩石的相互作用极其有限；因此，原始地幔特征有望得到较好的保存。本研究填补了地幔岩石学和矿物物理学之间的空白，并将解决以下问题：(1)UHP石榴石橄榄岩能否为上地幔岩石构造过程提供新的约束？(2)矿物的地球化学微分析对理解地幔-板块和岩石/流体相互作用过程有何帮助？(3)脱溶层在超高压矿物中的意义是什么？(4)石榴石橄榄岩的超高压变质作用还有哪些矿物学证据？苏鲁地体岩心和地表样品的主要逆冲作用包括：(1)石榴石橄榄岩的岩石学特征和变质演化；(2)地球化学对其来源和初始构造背景的制约；(3)放射性和稳定同位素组成，用于评价地壳/地幔与岩石/流体的相互作用；(4)采用各种方法测定锆石年龄，包括SHRIMP定年。这些综合的石化和地质年代学约束是评价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.