苏鲁超高压变质带胡家林超镁铁质岩成因及构造意义

The petrogenesis and tectonic implication of Hujialin ultramafic rocks in the Sulu ultrahigh-pressure metamorphic belt,eastern China

胡家林超镁铁质杂岩体产于苏鲁超高压变质带中部,纯橄岩和(石榴)单斜辉石岩呈不连续透镜体产于蛇纹石化橄榄岩中。纯橄岩遭受了部分蛇纹石化(烧失量=6.6%～13.2%),全岩富集强相容元素(Ni、Cr、Co)和Ir族PGE(IPGE;Ir、Os、Ru)及高IPGE/PPGE值,亏损Al、Ti、V,具高Mg~#橄榄石(Fo=91.7～92.4)和高Cr~#(0.68～0.76)尖晶石。纯橄岩高耐熔地球化学及矿物化学特征和古老的大陆岩石圈地幔相一致,表明其原岩来源于弧前地幔,代表了华北克拉通古老的大陆岩石圈地幔残留。(石榴)单斜辉石岩全岩呈相对低含量的强相容元素(Cr、Ni、Co)和IPGE,高含量的Al、Ti、V和流体迁移元素(Sr、Pb和Ba),球粒陨石标准化REE配分图呈明显"上凸"型,具低Mg~#橄榄石(Fo=76.6～76.8)和低Al_2O_3(<2.76%)和高SiO_2(54.56%～56.87%)的单斜辉石。全岩组成和矿物化学表明其原岩为俯冲带内超镁铁质火成堆晶岩,最初岩浆由地幔岩高程度部分熔融的熔体和俯冲带中富H_2O流体和/或熔体构成。(石榴)单斜辉石岩原岩曾被地幔流带入扬子大陆俯冲板片和上覆地幔楔之间的俯冲通道,经历了超高压变质作用和生成大量石榴石。(石榴)单斜辉石岩在折返过程中,与大陆岩石圈地幔楔剥离的蛇纹石化橄榄岩及纯橄岩相结合,形成超镁铁质杂岩体,整体被低密度的俯冲板片(主要由花岗质片麻岩和变质沉积岩组成)裹挟,折返至地壳浅部。

The Hujialin ultramafic complex in the central region of Sulu ultrahigh-pressure （UHP） metamorphic belt mainly consists of discontinuous lenses of garnet-bearing clinopyroxenite and dunite surrounded by marginal serpentinized peridotites. Dunite samples, which are partially hydrated （LOI=6.6%-13.2%）, contain high concentrations of compatible elements （Ni, Cr, Co） and IPGE with high ratios （up to 8.8） of IPGE/PPGE, low concentrations of Al, Ti, V in bulk rocks, olivine in dunite shows high Fo （91.7-92.4） and Cr-spinel displays high Cr# （0.68-0.76）. The high refractory geochemical characteristics and mineral chemistries of dunite samples are similar to that of the ancient subcontinental lithospheric mantle, suggesting that dunites originated from forearc mantle, and represented the relic of the ancient subcontinental lithospheric mantle. Garnet-bearing clinopyroxenites show relatively low concentrations of compatible elements （Ni, Cr, Co） and IPGE, high concentrations of Al, Ti, V, and fluid-mobile elements （Sr, Pb and Ba） in buck rocks. They show convex chondrite-normalized REE patterns, low Fo （76.6-76.8） in olivine, low Al2O3 （〈2.76%） and high SiO2 （54.56%-56.87%） in clinopyroxene. The bulk rock compositions and mineral chemistry suggest garnet-bearing clinopyroxenite is initially cumulate of subduction-related ultramafic melt, and the parental magma was formed by the melt from high degree of partial melting of mantle rocks and H2O-rich fluids/melt from subduction zone. The protoliths of garnet-bearing clinopyroxenite was individually incorporated into the subduction channel between the Yangtze subducted plate and overlying North China Craton by a mantle flow and then underwent UHP metamorphism. Garnet-bearing clinopyroxenite probably physically mixed with serpentinized peridotite lenses off-scraped from the overlying subcontinental lithospheric mantle （SCLM） wedge in the exhumation process, and both of them were coerced by low-density subducted slab （granitic gneiss and metamorphosed sedimentary rocks） and exhumed to the surface together.