Late Jurassic High-Pressure Metamorphism of Variscan I-Type Granitoids in the Northern Part of the Pelagonian Unit (Republic of North Macedonia)

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Rainer Altherr, Michael Hanel
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Abstract

The high-P/T metamorphic Pelagonian Unit in the Republic of North Macedonia comprises (1) a Variscan basement consisting of gneisses, schists and minor meta-mafic rocks which are all intruded by I-type granitoids and rare related dikes; (2) a metamorphosed sedimentary sequence of Permian to Lower Triassic age, and (3) a sequence of calcite and dolomite marbles resulting from Late Triassic to Middle Jurassic carbonate sediments. All these rocks underwent a common high-P/T metamorphism of Late Jurassic age. This paper deals with the metamorphism of the Variscan I-type granitoids which contained the igneous mineral assemblage plagioclase I + alkali feldspar I + quartz I + biotite I + titanite I + allanite I + zircon I + apatite I ± magnetite I. During Late Jurassic high-P/T metamorphism, these undeformed granitoids were thoroughly metamorphosed under isotropic pressure conditions as documented by undeformed granitic textures that are overgrown by metamorphic minerals such as garnet II, epidote II, and phengite II. Various, eventually metasomatic mineral reactions took place in different textural positions: (1) Former igneous plagioclase grains became completely transformed to Na-rich plagioclase IIa (An09–14) containing numerous small grains of epidote IIa and phengite IIa. Either this transformation was an allochemical one and was accompanied by the syn-metamorphic introduction of an aqueous fluid phase containing Fe, Mg and K or, alternatively, the more Ca-rich parts of plagioclase I became considerably sericitized before high-P/T metamorphism, and the resulting mixture of more Na-rich relic plagioclase with its sericite-rich domains became later metamorphosed under high-P/T conditions. In the first case, an aqueous phase is needed during metamorphism, while in the second case high-P/T metamorphism might have proceeded under H2O-undersaturated conditions; (2) igneous alkali feldspar I was changed to albite-poor orthoclase II or microcline II; (3) igneous Ti-rich biotite I reacted with plagioclase to metamorphic garnet II + Ti-poorer biotite II + titanite II + phengite II + quartz II ± epidote II ± rutile II, which is rimmed by Ttn II. At textural positions, where igneous plagioclase I was not available, igneous biotite I was transformed to Ti-poorer biotite II + titanite II ± ilmenite-hematite II; (4) during uplift, high-P/T metamorphic rutile II became marginally overgrown by titanite II ± ilmenite II; (5) igneous allanite I grains stayed unaltered, but when located near to former plagiocase I, they became partially rimmed by metamorphic epidote II. Equilibrium phase diagram calculations showed that the observed metamorphic paragenesis (plagioclase II + K-rich feldspar II + biotite II + garnet II + epidote II + phengite II + garnet II + quartz II + rutile II + titanite II) is only stable under H2O-unsaturated conditions. The I-type granitoids and their metamorphic country rocks were metamorphosed under high-P/T conditions of 1.3–1.5 GPa and 560–590°C.
北马其顿共和国佩拉戈尼亚单元北部瓦里斯坎I型花岗岩的晚侏罗世高压变质作用
北马其顿共和国的高P/T变质佩拉贡单元包括:(1) 由片麻岩、片岩和少量元杂岩组成的瓦里斯坎基底,这些岩石均由I型花岗岩和罕见的相关岩钉侵入;(2) 二叠纪至下三叠纪的变质沉积序列;(3) 由晚三叠纪至中侏罗纪碳酸盐沉积物形成的方解石和白云石大理岩序列。所有这些岩石都经历了侏罗纪晚期共同的高P/T变质作用。本文论述的是瓦里斯坎 I 型花岗岩的变质作用,这些花岗岩含有火成岩矿物组合斜长石 I + 碱长石 I + 石英 I + 生物辉石 I + 钛铁矿 I + 绿帘石 I + 锆石 I + 磷灰石 I ± 磁铁矿 I。在晚侏罗世高P/T变质作用期间,这些未变形的花岗岩在各向同性压力条件下发生了彻底的变质作用,未变形的花岗岩纹理被变质矿物(如石榴石II、附铁矿II和黝帘石II)所覆盖,证明了这一点。在不同的纹理位置发生了各种最终的变质矿物反应:(1)以前的火成岩斜长石颗粒完全转变为富含 Na 的斜长石 IIa(An09-14),其中含有大量的表长石 IIa 和辉长石 IIa 小颗粒。这种转变要么是一种分配化学转变,并伴随着含有铁、镁和钾的水液相的同步变质,要么是斜长石I中富含Ca的部分在高P/T变质之前就已被大量绢云母化,而由此产生的富含Na的遗迹斜长石与富含绢云母的混合体后来在高P/T条件下发生了变质。在第一种情况下,变质过程中需要水相,而在第二种情况下,高 P/T 变质作用可能是在 H2O 不饱和条件下进行的;(2)火成岩碱性长石 I 变为贫白云母正长石 II 或微斜长石 II;(3) 火成岩富钛黑云母 I 与斜长石反应生成变质石榴石 II +贫钛黑云母 II +榍石 II +黝帘石 II +石英 II ± 表长石 II ± 金红石 II,其边缘为 Ttn II。在没有火成岩斜长石 I 的纹理位置,火成岩生物黄铁矿 I 转化为钛坡勒生物黄铁矿 II + 钛铁矿 II ± 钛铁矿-赤铁矿 II;(4) 在隆起过程中,高 P/T 变质金红石 II 被榍石 II ± 钛铁矿 II 稍微覆盖;(5) 火成岩绿帘石 I 晶粒未发生变化,但在靠近原斜长石 I 的位置,部分晶粒被变质闪石 II 包覆。平衡相图计算表明,所观察到的变质副成因(斜长石 II + 富钾长石 II + 黑云母 II + 石榴石 II + 表长石 II + 正长石 II + 石榴石 II + 石英 II + 金红石 II + 钛铁矿 II)只有在 H2O 不饱和条件下才是稳定的。I 型花岗岩及其变质乡村岩是在 1.3-1.5 GPa 和 560-590°C 的高 P/T 条件下变质的。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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