浙西南遂昌-大柘地区石榴角闪二长片麻岩成因及变质演化

Genesis and Metamorphic Evolution of Garnet Amphibolite Monzogneiss from Suichang-Dazhe Region in Southwestern Zhejiang Province

浙西南遂昌-大柘地区八都群中分布一套含石榴石、角闪石的二长片麻岩,发育典型石榴石"白眼圈"反应结构,但其成因及变质演化目前尚不明确.运用矿物X射线电子探针微区分析、LA-ICP-MS锆石U-Pb定年并结合全岩主微量元素进行了研究.识别出3个阶段的变质矿物共生组合:进变质阶段(M_1)矿物组合为石榴石变斑晶内部的包裹体矿物石榴石+角闪石+斜长石+石英(Grt_1+Amp_1+Pl_1+Q),变质峰期矿物组合(M_2)为石榴子石变斑晶幔部和基质矿物钾长石+斜长石+黑云母+石英(Grt_2+Kf_2+Pl_2+Bt_2+Q),退变质阶段矿物组合(M_3)为"白眼圈"后成合晶角闪石+斜长石+黑云母+钛铁矿(Amp_3+Pl_3+Bt_3+Ilm).矿物地质温压计和相平衡模拟估算的3阶段P-T条件分别为:进变质阶段为600~700℃/0.60~0.65GPa,变质峰期为800~820℃/0.94~0.96GPa,退变质阶段为550~700℃/0.56~0.71GPa,变质程度达到麻粒岩相,变质作用PTt轨迹显示顺时针型式,具有近等温降压(ITD)特征,暗示其经历了地壳加厚和快速折返过程.岩石地球化学特征显示其原岩为准铝质A型花岗岩,含有少量幔源组分,形成于造山后的陆壳拉张环境;锆石U-Pb定年呈现二阶段年龄特征,成岩年龄为1.83~1.85Ga,表明石榴角闪二长片麻岩的原岩形成于古元古代,变质年龄为220~230Ma,显示了印支期变质作用对古元古代花岗岩的改造,是浙西南地区对印支期古太平洋板块向华南板块俯冲过程的响应,也为浙西南地区印支期造山作用提供了新的证据.

There is a set of garnet amphibolite monzogneiss which developed typical garnet＂white eye socket＂structure from the Badu Group in Suichang-Dazhe region,southwestern Zhejiang Province.However,its genesis and metamorphic evolution remain unclear.In this study,mineral X-ray microprobe microanalysis,LA-ICP-MS zircon U-Pb dating and whole-rock major and trace element analyses were carried out to address these questions.Three stages of mineral assemblages have been identified：the prograde metamorphic assemblages（M1）are mineral inclusions garnet＋amphibole＋plagioclase＋quartz（Grt1 ＋Amp1＋Pl1 ＋Q）preserved in the garnet porphyroblasts,the peak metamorphic assemblages（M2）are represented by the＂garnet-mantles＂and the matrix minerals garnet＋K-feldspar＋plagioclase＋biotite＋quartz（Grt2 ＋Kf2 ＋Pl2 ＋Bt2 ＋Q）,whereas the retrograde assemblages（M3）are composed of amphibole＋plagioclase＋biotite＋ilmenite（Amp3＋Pl3＋Bt3＋Ilm）symplectites rimming the garnet porphyroblasts.Thermobarometric computation and phase equilibrium modeling show that the metamorphic conditions of the garnet amphibolite monzogneiss are 600-700 ℃/0.60-0.65 GPa for the M1 stage,800-820℃/0.94-0.96 GPa for the metamorphic peak M2 stage,and 550-700℃/0.56-0.71 GPa for the retrograde M3 stage,respectively.Metamorphic grade reached granulite facies and metamorphism PTt paths are characterized by clockwise pattern involving the nearly isothermal decompression（ITD）segments,indicating crustal thickening and rapid exhumation process.Petrogeochemistry shows that the protolith of monzogneiss is metaluminous A-type granite with some mantle-derived components which formed in continental crust extensional environment during post orogeny.LA-ICP-MS zircon U-Pb dating indicates two-stage age characteristics,in which the diagenetic age of 1.83-1.85 Ga suggests the protolith of monzogneiss formed in the Paleoproterozoic and the metamorphic age of 220-230 Ma shows the reworking of Indosinian metamorphism to Paleoproterozoic granite.It is the response to the subduction of Indo Pacific plate into South China plate during Indosinian,which provides new evidence for Indosinian orogeny in the southwestern Zhejiang.