中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体

中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。

Ultrahigh-pressure mineral inclusions preserved in zircons separated from eclogite and its country-rocks in the main drill hole of Chinese Continental Scientific Drilling Project (0～4500 m)

The main drill hole of Chinese Continental Scientific Drilling (CCSD-MH) Project is located in Donghai, southwestern Sulu terrane. The metamorphic rocks in the depth intervals of 0-4500 m revealed by CCSD-MH consist mainly of eclogite, amphibolite, paragneiss and orthogneiss, with thin layers and blocks of ultramafic rocks. All analyzed core samples (except eclogite) from CCSD-MH are overprinted by amphibolite-facies retrograde metamorphism, with characteristic amphibolite-facies mineral assemblages. Inclusions of index ultrahigh-pressure (UHP) minerals in zircon separates from 121 core samples from CCSD-MH were identified using laser Raman spectroscopy, cathodoluminescence images and electron microprobe analysis. The UHP mineral inclusion assemblages are characterized by coesite garnet, coesite garnet omphacite rutile and coesite phengite apatite in zircons from phengite-bearing rutile quartz eclogite. In the zircons separated from biotite epidote amphibolite, the UHP mineral assemblages consist mainly of coesite garnet omphacite, coesite garnet phengite and coesite omphacite rutile. These data indicate that the amphibolite is the retrogressive product of UHP eclogite. In the zircons from garnet epidote biotite two-feldspar gneiss, the index UHP mineral assemblages are characterized by coesite phengite and coesite garnet. However, in garnet biotite amphibole albite gneiss, coesite together with jadeite garnet apatite, jadeite phengite apatite and garnet apatite as index UHP mineral assemblage inclusions are commonly preserved in overgrowth zircon domains. In the zircons separated from orthogneiss, the UHP minerals are characterized by coesite, coesite phengite, coesite kyanite apatite and coesite kyanite titanite. Similarly, zircons separated from outcrops in Donghai and Linshu area, including parangiess, orthogneiss, marble, kyanite-bearing quartzite, schist, amphibolite and eclogite, also contain abundant coesite-bearing UHP mineral inclusions. These evidences suggest that the volumetrically continental materials, mainly consisting of protolithes of eclogite and country rocks, commonly occurred deep-subduction, experienced ultrahigh-pressure metamorphism, and then rapidly returned to the earth 's surface. In generally, the distribution of mineral inclusions in zircons and cathodoluminescence images of zircons from same zircon sample displays distinct zoning from core to rim. Some zircon grains can be genetically correlated with inherited ( detrital) core, UHP metamorphic mantle and retrogressive rim, and others with UHP metamorphic core and mantle, and retrogressive rim. These data indicate that the complex growth-evolution histories were preserved in zircons from all metamorphic rocks in CCSD-MH. Cathodoluminescence images reveal irregular zoning patterns and various thickness of cores, mantles and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith, ultrahigh-pressure and retrogressive age dating.