摘要：
Garnet amphibolites hidden abundant metamorphic textures are very significant in revealing the tectonic-thermal evolution of the Paleoproterozoic orogenic belts.The Sanjiazi garnet amphibolites,which located in the northern Liaodong Penisula from the Jiao-Liao-Ji belt,have been identified mainly as deformed dyke within the Paleoprterozoic Bt-bearing paragneisses from the South Liaohe Group,and parallel the regional east to west foliation.The garnet amphibolites cantain index'White eye'texture and show three distinct metamorphic mineral assemblages:a pre-peak assemblage represented by fine-grained inclusion assemblage (M1) of quartz + amphibole + ilmenite + magnetite within the garnet porphyroblasts,peak high-presurre (HP) amphibolite-facies assemblage (M2) that consists of garnet + amphibole + clinopyroxene + plagioclase (An =44 ～ 45) + quartz + ilmenite,a post-peak decompression assemblage (M3) that is characterized by symplectites of amphibole + plagioclase (An =64 ～ 90) + quartz + ilmenite + magnetite.Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrained the P-T conditions of the M1,M2 and M3 assemblages to P =0.68 ～ 0.72GPa and T =570 ～ 600°C,P =0.98 ～ 1.01GPa and T =690 ～ 710°C,and P =0.52 ～ 0.61GPa and T =670 ～ 700°C,respectively.An integrated study involving scanning electron microscope analysis of mineral inclusions,cathodoluminescence imaging,and LA-ICP-MS U-Pb dating of zircons showed that the protolith ages of the garnet amphibolites are mainly 2.17 ～2.06Ga,the timing of the peak HP amphibolite-facies metamorphism ranges from 1.96Ga to 1.94Ga,and the medium-to low-pressure amphibolite-retrogression occurred mainly at 1.92 ～ 1.83Ga,as recorded by zircon grains that contain inclusions of amphibole + plagioclase (An =60 ～ 90) + quartz + apatite.Comprehensive petrographic,mineralogical,and geochronological investigations of the garnet amphibolites defined a clockwise P-T-t path involving tempreture-and pressure-increasing and near-isothermal decompression from M1 to M3.This P-T-t path is related to the tectonic thickness by the Paleoproterozoic collisional orogenesis of the Jiao-Liao-Ji belt.%石榴斜长角闪岩常蕴含着丰富的变质反应结构,对古元古代造山带构造热演化过程研究具有十分重要的科学意义.胶-辽-吉带辽东半岛北部三家子地区出露的石榴斜长角闪岩,主要以岩脉的形式顺层侵入至南辽河群(里尔峪岩组)含黑云母的斜长片麻岩之中.三家子石榴斜长角闪岩以发育典型的“白眼圈”结构为特征,并保留了三个不同演化阶段矿物组合,峰前矿物组合为分布于石榴子石内部细粒包体矿物,主要包括角闪石+石英+磁铁矿+钛铁矿(M1);峰期矿物组合为石榴子石+角闪石+单斜辉石+斜长石(An =44 ～45)+石英+钛铁矿(M2);峰后退变质阶段矿物组合为角闪石+斜长石(An =64～90)+石英+钛铁矿+磁铁矿+榍石(M3).相平衡模拟与矿物对地质温压计算表明,三家子石榴斜长角闪岩峰前变质阶段(M1)、峰期变质阶段(M2)与峰后退变质阶段(M3)稳定的P-T条件分别为:P=0.68 ～0.72GPa、T=570 ～ 600°C,P=0.98～1.01 GPa、T=690 ～710°C,P=0.52 ～0.61GPa、7=670 ～ 700°C.锆石矿物包体扫描电镜分析、锆石CL图像分析与锆石LA-ICP-MS U-Pb定年的综合研究表明,三家子石榴斜长角闪岩原岩形成时代为2.17～2.06Ga之间;近峰期高压角闪岩相变质的时代为1.96～1.94Ga;峰后近等温减压中-低压角闪岩相退变质时代为1.92～1.83Ga,记录该组年龄的变质锆石微区含有典型的中-低压角闪岩相矿物包体(角闪石+斜长石(An =60～90)+石英+磷灰石).综合以上分析,本文初步认为三家子石榴斜长角闪岩具有顺时针P-T-t轨迹,其主要特点是从M1到M3表现为升温升压的变质过程,并达到变质作用的峰期阶段,峰后开始表现为近等温减压的变质过程,随后可能表现为冷却降温的退化变质过程.因此,它们与胶-辽-吉带古元古代造山作用有关的构造增厚作用有密切关系.
