华北东南缘夹沟前寒武纪镁铁质下地壳包体的变质演化和P-T轨迹

METAMORPHIC EVOLUTION AND P-T PATHS OF THE PRECAMBRIAN LOWER-CRUST MAFIC XENOLITHS FROM JIAGOU AREA AT THE SOUTHEASTERN MARGIN OF THE NORTH CHINA CRATON

华北东南缘中生代闪长斑岩中深源麻粒岩包体或捕掳体属于前寒武纪下地壳岩石。针对石榴斜长角闪岩和石榴麻粒岩两类镁铁质下地壳包体进行详细的野外观察、岩相学观察和矿物的电子探针分析,结果表明石榴子石呈微弱的环带结构,石榴斜长角闪岩中,由核部至边部X_(Mg)为0.25~0.32,逐渐升高;XCa为0.27~0.19,逐渐降低。而在石榴麻粒岩中,由核部至边部XMg和XCa均呈现为逐渐降低,分别为0.25~0.19和0.28~0.17。两类岩石中,以包体形式产出的单斜辉石和基质单斜辉石的核部具有较高的钠和铝含量,基质单斜辉石的边部钠和铝的含量相对较低,斜长石以中长石为主,不同产状的斜长石钙含量变化很大,以包体形式存在的斜长石钙含量较高,其成分为An27Ab70Or3(石榴斜长角闪岩)和An29Ab69Or3(石榴麻粒岩);以基质变斑晶和后成合晶产出的斜长石钙含量较低,分别为An5-36Ab62-95Or0-2和An6-48Ab50-94Or0-1(石榴斜长角闪岩),An5-51Ab48-94Or1-2和An32-79Ab21-68Or0-3(石榴麻粒岩)。不同类型角闪石的钛含量有较大差别:棕色角闪石的钛含量较高,w(TiO2)一般为0.53%~1.44%;而绿色角闪石钛含量较低,w(TiO2)一般为0.03%~0.38%。研究结果证明夹沟镁铁质下地壳包体经历了高压麻粒岩相、低压麻粒岩-高角闪岩相、角闪岩相和绿片岩相变质过程。传统地质温压计限定的前三个变质阶段的温压条件分别为824℃~844℃/1.5GPa^1.9GPa,667℃~730℃/1.0GPa^1.3GPa和609℃~646℃/0.4GPa^0.8GPa,显示近等温减压的顺时针P-T-t轨迹,指示它们形成于晚古元古代板片俯冲或弧-陆碰撞的构造环境。因此,研究区属于胶-辽-吉带的西延并共同构成了华北东部一条古元古代碰撞造山带。

The deep-seated granulite xenoliths in the Mesozoic dioritic porphyry in the Xuzhou- Suzhou region at the southeastern margin of the North China Craton （NCC） belong to the Pre- cambrian lower-crustal or mantle-derived rocks. These xenoliths include garnet amphibolite,gar- net granulite,garnet-bearing plagioclase-amphibole gneiss, granitoid gneiss and so on. The Pre- cambrian lower-crustal xenoliths are the important window or rock ＂probe＂ for the formation and evolution of the lower crust. It provides an excellent natural laboratory and an important way to reveal the early continental crust growth and metamorphic evolution. Combined field investiga- tion,petrologic observation and electron microprobe analyses of minerals on two types of mafic xenoliths （garnet amphibolite and garnet granulite） are carried out in order to understand the for- mation and evolution of the Precambrian crust （especially the lower-crust） in the southeastern margin of the NCC. It shows that the garnet has a weak zonal structure in composition, and the XMg increases from 0. 25~0. 32 from the core to the rim for the garnet amphibolite,while Xc. de- creases from 0.27~0.19. However,in garnet granulite,XMg and Xca decrease gradually from 0.25 t0. 19 and 0. 28~0. 17 from the core to the rim,respectively. Among the two types of marie xen- oliths,the clinopyroxene in xenoliths and matrix have higher sodium and aluminum content,while the rims of clinopyroxene in matrix have relatively low sodium and aluminum content. Plagioclase is mainly oligoclase,and plagioclase of different origins varies greatly in calcium content. Plagio- clases exist in the form of inclusions have high Ca contents with composition of An27 Ab70Or3 （gar- net amphibolite） and An29 Ab69 Ora （garnet granulite） plagioclase in the form of porphyroblast in matrix have lower calcium content with composition of An5-36 Ab62-95 Or0-2 and An6-48 Ab50-94 Or0.1 （garnet amphibolite）,An5-51 Ab48-94 Or1-2 and An32-79 Ab21-68 Or0-3 （garnet granulite）. Different types of amphiboles have a large difference in titanium content, while the brown amphibole has a high content of titanium [w（TiO2） 0.53%~1.44 %], whereas the green amphibole has very low or al- most no titanium content [w（TiO2） is 0. 03~0. 38%]. It shows that the Jiagou mafic xenoliths have undergone high pressure granulite-facies, low-pressure granulite-high amphibolite-facies, am- phibolite-facies and greenschist-facies metamorphic processes. Their P-T conditions have been es- timated by using traditional thermobarometry. It shows that the range of high-pressure granulite- facies metamorphism is 824 ℃~844 ℃/1.5 GPa-1. 9 GPa;the range of low-pressure granulite- high amphibolite-facies is 667 ℃~730 ℃/1.0 GPa-1. 3 GPa and the range of amphibolites-facies metamorphism is 609 ℃ ~646 ℃/0.4 GPa^0.8 GPa. On the basis of the previous zircon dating data,it is indicated that the lower-crust mafic xenoliths in the region experienced a clockwise P-T- t evolution with near-isothermal decompressional processes,under a tectonic setting of suhduction or arc-continent collision in the late Paleoproterozoic. Therefore, the studied region is the western extension of the Jiao-Liao-Ji belt,which altogether made up a Paleoproterozoic collisional orogen as a whole in the eastern block of the NCC.