恒山东段新太古杂岩两种PTt轨迹的确定及其意义

Determination and Significance of Two PTt Paths in the Neoarchean Complex of Eastern Hengshan

恒山新太古杂岩指董庄表壳岩和侵入其中的土岭—林场灰色片麻岩体。董庄基性麻粒岩和土岭—林场岩体中的斜长石榴辉石岩包体经历了两种不同的PTt轨迹,前者为逆时针轨迹,表现为压力滞后快速升温的进变质轨迹和近等压降温的退变质轨迹;后者为顺时针轨迹,表现为温度滞后快速增厚的进变质轨迹和近绝热减压的退变质轨迹。本文探讨了该杂岩的形成和演化过程。

The Neoarchean complex in eastern Hengshan is composed of Dongzhang supracrustal rocks and Tuling--Linchang TTG gneisses. Mafic granulites ofDongzhuang Formation contain two--stages of mineral assemblages, with thesecond assemblage (M_3) of Hb+Pl mantling the first assemblage (M_2) of OPx+CPx+Pl±Gt and often. occurring as symplectic coronas passing between OPxand Gt. After the earliest metamorphism (M_1), the plagioclase--garnet--twopyroxene rocks occurring as inclusions in the TTG gneisses formed two--stagesymplectic coronas around garnet. An outer corona of Hb+Pl mantles a finer-grained inner corona of OPx+Pl. Preferred P--T estimates for M_1 based on garnet--OPx pairs in the inclusionsof Tuling-Linchang TTG gneisses are P about 1.1--1.5 GPa at 1108°K. Thereexist some differences in P-T conditions for M_2 and M_3 between the inclusionsand the host rocks of Tuling-Linchang TTG gneisses, with P--T for M_2 and M_3being respectively 1.0 GPa at 1148±25°K and 0.4 GPa at 923±50°K in theinclusions, and 0 .9 GPa at 1123±20°K and 0.4 GPa at 908±45°K. in DongzhuangFormation. According to pressure, temperature and textures indicating mineral reac-tions, they have experienced different evolutionary Processes. The PTt path ofthe inclusions is a‘clockwise' path in which the barometric maximum precedesthe thermal maximum, and initial retrograde path has large initial dP/dT,exhibiting isothermal decompression(ITD), whereas the PTt path of Dong-zhuang mafic granulites is an 'anticlockwise' path in which heating occurs Priorto during the thickening, and initial retrograde path has small positive dP/dT,exhibiting isobaric cooling(IBC). Such differences are of great importance indiscussing the evolution of the Neoarchean complex in this area.