豫西秦岭杂岩变质带的分布及主期变质时代的限定

Metamorphic zones and constraining on timing of the major metamorphism event in the Qinling Complex,western Henan Province

豫西秦岭杂岩中变质分级可呈与造山带大致平行的带状分布,从两侧向中心变质级别升高,尤其南侧分带明显:由南向北,依次为黑云母带-石榴子石带-蓝晶石带-夕线石带,直至斜方辉石带,而不是整体上经历了麻粒岩相变质作用。局部发生的麻粒岩相变质未见明显向角闪岩相变质转化的退变结构。通过几种岩石的锆石LA-MC-ICPMS测年研究,多数样品中的锆石经受了后期强烈的改造,同位素体系或多或少被重置。尽管如此,侵位花岗岩和伟晶岩年龄限定了主期变质作用的时代应老于484±3Ma,并可能与早期的榴辉岩相变质作用在演化上有联系。主期变质(不包括榴辉岩相变质)性质与经典的巴罗式变质带可以对比;此外,研究区未经历明显的地壳增厚,与高喜马拉雅结晶岩系类似,秦岭杂岩可能经历了中、下地壳物质沿隧道流上升过程。

Through detailed field and indoor observation and analyses,several metamorphic zones parallel to the orogenic belt can be discerned in the Qinling Complex,western Henan Province. The metamorphism intensity of the zones increase gradually from both the southern and northern margins to the core of the complex,and the southern part is especially distinguished in the northward increasing grades of biotite,garnet,kyanite and sillimanite zones in the metapelites,and the highest orthopyroxene zone in the metabasic rock in the central. The complex is not wholly granulite facies metamorphosed as conventionally considered,and no substantial retrograde textures from granulite to amphibolite facies metamorphism have been noticed. LA-MC-ICPMS data of zircons from various rocks demonstrated that many zircons have experienced severely intensive reworking and resetting in isotopic system.Nonetheless,the authors manage to constrain the major metamorphism event older than 484 ± 3Ma through the intrusive granite and pegmatite in the gneisses,and the event might be related with the earlier eclogite facies metamorphism. As regards to the feature of major metamorphism（ except the eclogite facies）,the complex is well compared with the typical Barrowian metamorphism and no substantial crustal thickening can be deduced. The Qinling Complex is rather similar to that of the High Himalayan Crystalline rocks in that the channel flow mechanism is the quite possible in the geological process.