北大别混合岩类型及其成因

ROCK TYPES OF MIGMATITES IN THE NORTH DABIE AND THEIR PETROGENESIS

大别造山带北大别超高压变质构造单元中广泛发育混合岩。基于对罗田和岳西穹隆中混合岩的野外观察、岩相学、矿物化学和锆石LA-ICP-MS U-Pb定年系统工作,发现北大别混合岩主要分为中等深熔混合岩和高度深熔混合岩两种类型。中等深熔混合岩为叠层状混合岩和膨胀结构混合岩;高度深熔混合岩为眼球状混合岩和析离体状混合岩。锆石U-Pb同位素定年结果表明,混合岩中新生锆石年龄范围为125 Ma^138 Ma,加权平均年龄为130.7 Ma±1.8 Ma;两个继承锆石的年龄分别为602.8 Ma±16.8 Ma和667.3 Ma±17.6 Ma;混合岩叠层状暗色体中锆石年龄连续且分布集中,加权平均年龄为703 Ma±10 Ma,代表原岩的形成年龄。北大别混合岩的原岩为新元古代的岩石,其混合岩化作用发生在140 Ma至125 Ma期间。不同浅色体中斜长石的含量和牌号差异显著,反映了浅色体形成于不同演化程度的熔体结晶。斜长石-角闪石温压计估算结果显示,混合岩形成的温压条件为723℃~768℃和370 MPa^520 MPa,对应于中上地壳环境。混合岩的成因机制以长英质片麻岩水饱和条件下的富水熔融为主。其反应为:黑云母+石英+斜长石+水=角闪石+斜长石(残留)+花岗质熔体。少数混合岩的成因机制为角闪片麻岩中角闪石的脱水熔融。其反应为:角闪石+斜长石+石英=单斜辉石+富水熔体。

Migmatites in the north Dabie ultrahigh-pressure metamorphic terrane （NDT） of the Dabie orogen are systematically studied by means of field observations, petrographical analysis, mineral chemistry and zircon LA-ICP-MS U-Pb dating. It shows that the migmatites can be divided into two main types, the metatexite migmatites and diatexite migmatites. The former are composed of stromatic-structured metatexite migmatites and dilation-structured metatexite migmatites. And the latter are composed of schollen-structured diatexite migmatites and schlieren-structured diatexite migmatites. Zircon U-Pb isotopic dating reveals that the magmatic zircon age of the migmatite ranges from 125 Ma to 138 Ma,with a weighted mean age of 130.7 Mail. 8 Ma. And two inherited zircons yield about 602.8 Ma± 16.8 Ma and 667.3 Ma±17. 6 Ma ages. The stromatic melanosome zircon age is continuous and concentrated,with a weighted average age of 703 Ma±10 Ma,representing the age of the protolith. Therefore,the protolith of north Dabie migmatites is Neoproterozoic in age and the migmatization occurs during 140 Ma to 125 Ma. The differences in the contents and An values of plagioclases in the leucosomes are resulted from the evolution degrees of melting from which the leucosomes crystallized. Based on the plagioclase-hornblende thermobarometers,the forming temperature and pressure of migmatites range from 723 ℃ to 768℃ and 370 MPa to 520 MPa respectively,corresponding to the environment of middle and upper crust. Petrogenesis analysis indicates that most migmatites are resulted from water-fluxed melting under a condition of water-saturated felsic gneiss,and the reaction is Biotite ＋ Plagioclase ＋ Quartz＋ Fluid= Granitic melt＋ Amphibole＋ Residual plagioclase. Besides, some of the migmatites are formed by dehydration partial melting of hornblende, with the reaction as Amphibole＋ Plagioclase＋ Quartz = Clinopyroxene＋ H2 O-bearing fusant.