北秦岭西部榴闪岩的发现与变质作用过程:锆石年代学和Hf-O同位素制约

Discovery of garnet amphibolite in western part of the North Qinling orogenic belt and its inferred metamorphic process: Constraints from zircon geochronology and Hf-O isotopes

秦岭造山带是由多期俯冲-碰撞造山和随后多阶段隆升成山形成的复合型造山带。近年来一个突破性进展是在北秦岭东部即陕西-河南交界地区以及桐柏地区的秦岭群高级变质岩中发现存在大量退变榴辉岩和高压麻粒岩,其中部分退变榴辉岩锆石包裹体中出现柯石英。这些发现证明北秦岭东部和桐柏地区的确在480~500Ma发生过深俯冲作用,随后造山带抬升形成退变榴辉岩。那么,北秦岭西部是否存在类似的(深)俯冲作用?本文首次在北秦岭西部宝鸡眉县地区发现了榴闪岩。该榴闪岩主要由石榴石和角闪石组成,含少量石英、透辉石和绿帘石。石榴石变斑晶都具有明显由退变质扩散作用形成的环带结构,绿帘石变斑晶具有与榴辉岩中绿帘石大致相同的XFe、CaO和Al2O3含量。同时,石榴石变斑晶核部还含有Amp+Cpx+Qtz包裹体,该包裹体矿物组合暗示其退变前可能是榴辉岩,而榴闪岩是在流体活动过程中退变质作用形成的。榴闪岩中锆石的离子探针SIMS定年获得很好的U-Pb谐和年龄(443.9±1.1Ma)和206Pb/238U的加权平均年龄(443.2±2.5Ma)。锆石的低Th和U含量和非常高Th/U比值(0.47~1.66)说明其并非是在"干"体系下形成的进变质锆石,应该是在流体强烈活动下形成的或是进变质锆石被退变质过程完全重置。因此,443Ma应该是榴闪岩的形成年龄即记录退变质作用发生的时间。这些锆石具有亏损的Hf同位素和类似地幔的氧同位素组成。因遭受退变质作用的强烈影响,其εHf(t)值和δ18O值有较宽的分布范围(εHf(t)=2.4~14.7;δ18O=4.25‰~6.39‰),其亏损地幔模式年龄为0.48~0.95Ga。所以,锆石的Hf-O同位素特征指示榴闪岩的原岩主要来自新元古代时期的亏损地幔来源的玄武质岩石。结合北秦岭东部地区榴辉岩发生退变质作用时代间隔可以大致判断北秦岭西部的榴闪岩的峰期变质时代可能为470~480Ma。由此发现,北秦岭东西部早古生代俯冲-碰撞事件时限上存在差异,即北秦岭早古生代俯冲-碰撞作用沿商南-宝鸡一线可能是自东向西发生的。

The Qinling orogenic belt is a complicated orogenic belt which was resulted from multiple plate subduction-collision orogeny and subsequent several-stages mountain-building uplifting. A breakthrough progress in recent years is the discovery of occurrence of abundant retrograde eclogites and high-pressure granulites in high-grade metamorphic rocks of the Qinling Group in the eastern part of the North Qinling belt,i. e. the Shaanxi-Henan border area and the Tongbai region,in which zircons from parts of retrograde eclogites contain coesite inclusion. These discoveries demonstrated that deep subduction during 480 - 500 Ma indeed happened in the eastern part of the North Qinling belt and the Tongbai region,subsequent uplifting of the orogenic belt produced the retrograde eclogite. Now,the question is whether or not similar deep subduction happened in the western part of the North Qinling belt？This paper first found the body of the garnet amphibolite in the western part of the North Qinling belt in Meixian,Baoji City. The garnet amphibolite is dominantly composed of garnet and amphibole with minor quartz and diopside. Garnet porphyroblasts show obviously compositional zoning yielded by the diffusion during the retrograde metamorphism. The epidote porphyroblasts show similar compositions in XFe,CaO and Al2O3 contents as those epidotes from eclogites. Meantime,garnet porphyroblast contains Amp ＋ Cpx ＋ Qtz inclusions. Such a mineral inclusion assemblage indicates that it could be eclogite prior to the retrograde metamorphism and the garnet amphibolite was formed with the fluid activity during the retrograde metamorphism. Zircon SIMS geochronology yielded an excellent UPb concordant age of 443. 9 ± 1. 1Ma and206Pb/238 U weighted mean age of 443. 2 ± 2. 5Ma. Low Th and U abundances and extremely high Th/U ratios（0. 47 - 1. 66） in zircons illustrate that these zircons were not the prograde metamorphic zircons produced during the＂dry＂system,but should be formed during the intense fluid activity or metamorphic zircons completely reset during the retrograde metamorphism. Therefore,the 443 Ma is interpreted as the retrograde metamorphic age of the garnet amphibolite,i. e. it records the event of the retrograde metamorphism. These zircons display a depleted Hf isotopic feature and normal mantle-like oxygen isotopic compositions although they have a wide range in ε8Hf（t）（2. 4 - 14. 7） and δ1O = 4. 25‰ - 6. 39‰） because of different degree affection during the retrograde metamorphism. The Hf isotopes yield roughly tDMages of 0. 48 - 0. 95 Ga. Thus,zircon Hf-O isotopic features point out that the protolith of the garnet amphibolite were Neoproterozoic basaltic rocks from the depleted sources. Compared with the time interval between the prograde and retrograde of the eclogites in the eastern part of the North Qinling orogenic belt,we can speculate that the peak metamorphism of the garnet amphibolite was roughly 470 - 480 Ma in the western part of the North Qinling orogenic belt. In summary,the time difference occurred in the Early Paleozoic subduction-collision events between the eastern and western parts of the North Qinling orogenic belt,namely Early Paleozoic subduction-collision events in the North Qinling orogenic belt along the Shangnan-Baoji line may occur from east to west.