徐淮地区中生代深部地壳的物质组成及热状态——深源捕虏体证据

COMPOSITION AND THERMAL STATE OF MESOZOIC DEEP CRUST IN XU-HUI AREA: EVIDENCE FROM DEEP-SEATED XENOLITHS

徐淮地区中生代侵入杂岩中含有丰富的深源岩石捕虏体,包括榴辉岩类、单斜辉石岩、角闪岩类和片麻岩类。深源捕虏体的岩相学研究表明,它们经历了早期的榴辉岩相变质和晚期的角闪岩相退化变质,然后被寄主岩浆所捕获。捕虏体中矿物平衡温压估算结果显示,榴辉岩相变质的温压条件为:T=657～992℃,P=1.33～2.78GPa,所反映的地温梯度介于克拉通地温(40mW/m2)和大洋地温(60mW/m2)之间;晚期角闪岩相退变的温压条件为:T=523～878℃,P=0.5～1.02GPa,这与中国东部新生代地温梯度相一致。本区中生代深部地壳主要由一套混杂堆积岩所构成,包括退变榴辉岩、退变石榴辉石岩、角闪岩类和片麻岩类。该区深源捕虏体的P-T演化暗示中生代早期华北地块东部深部地壳和/或岩石圈地幔曾存在一次重要的加厚过程;苏鲁造山带的形成可能是扬子地块沿北西方向俯冲于华北地块之下的产物;中生代郯庐断裂带并不存在巨大左行平移。

Abundant deep seated xenoliths such as eclogite, granet clinopyroxenite, clinopyroxenite, amphibolite and gneiss can be found in the Mesozoic intrusive complex from the Xu Huai area. The petrographical data of the deep seated xenoliths indicate that they could be subjected to earlier eclogite facies high pressure (HP) and ultra high pressure metamorphism (UHPM) and late amphibolite facies retrogressive metamorphism, then caught by the host magma. The P- T estimated results for the deep seated xenoliths show that earlier eclogite facies metamorphism happened under T=657 992℃ , P=1.33 2.78 GPa, which is similar to the geothermal gradient between the craton (40 mW/m2) and oceanic (60 mW/m2) geotherm, and that the P- T conditions of the late amphibolite facies retrogressive metamorphism are T=523 878℃ , P=0.5 1.02 GPa, respectively, which is of characteristic of Cenozoic geotherm in eastern China. Mesozoic deep crust in this area could be occupied by a suite of hybrid cumulates including eclogite, granet clinopyroxenite, clinopyroxenite, amphibolite and gneiss. The P- T evolution of the deep seated xenoliths implies that the continental crust and/or lithospheric mantle thickening could take place in eastern part of the North China block at early Mesozoic, that Su Lu orogen could be resulted from the subduction of the Yangtze block beneath the North China block in NWW direction, and that no huge strike slip occurred in the Tan Lu fault zone in the Mesozoic.