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松树沟蛇绿岩是东秦岭构造带内出露规模最大的超镁铁—镁铁质杂岩体。地质填图证实,该区存在结构十分复杂的大型逆冲推覆构造系统,其中北区松树沟超镁铁质主岩体内发育的高角度逆冲断层系向北拆离;而包括富水杂岩在内的南区镁铁质及超镁铁质岩块均向南大规模逆掩;剖面上总体构成不对称(向南滑脱为主)的“背冲型”样式。松树沟逆冲推覆构造研究对恢复蛇绿岩组合层序及其古构造环境具有重要的地质意义。 相似文献
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东秦岭商丹断裂带中松树沟超镁铁质岩系地球化学研究 总被引:4,自引:0,他引:4
东秦岭商丹断裂带中松树沟超镁铁质岩体由两类橄榄岩组成。其中,第Ⅱ类橄榄岩的化学成分非常局限,高度富集Co,Cr,Ni,贫乏Ti,V,Cu,Y;亏损不相容元素(K+Na,Al.Sr.Ba.Nd,Ga和REE);稀土元素和同位素特征揭示它代表本区早期地幔分离部分熔融的残余部分,并作为本区元古代洋壳蛇绿岩底部的变质橄榄岩。而第Ⅰ类橄榄岩的岩石化学成分,微量元素和同位素表明,它不与第Ⅱ类橄榄岩同源,属橄榄石堆积晶渣和原生岩浆的混合物,具有洋壳蛇绿岩中堆晶杂岩的性质,两类橄榄岩的成因和演化历史相互独立,通过构造侵位伴生,并共同代表本区元古代洋壳残片。 相似文献
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The Qinling orogenic belt experienced multiple phases of orogenesis during the Palaeozoic. Unraveling the timing and P–T conditions of these events is the key to understanding the convergence processes between the South China and the North China Blocks. The Songshugou Complex, located in the southern part of the North Qinling orogenic belt, has registered multistage metamorphism in Palaeozoic, and thus potentially provides insights into the tectonic evolution of the Qinling orogenic belt. In this study, three metabasic rocks (a garnet pyroxenite, a garnet amphibolite and a gneissic amphibolite) from the Songshugou Complex were selected for petrological study and zircon and titanite U–Pb dating. Our results show that the metabasic rocks experienced three metamorphic events during the Palaeozoic. The first metamorphic event (M1) is characterized by high pressure conditions. Two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample. They yielded Ti-in-zircon temperatures of 660–851 °C at ∼12.0 kbar and a weighted mean age of 498 ± 15 Ma, providing the constraints on the temperature and timing of prograde or peak metamorphism (M1-1). Zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave U–Pb ages of 494 ± 9 Ma and 484 ± 4 Ma, and Ti-in-zircon temperatures of 793 ± 33 °C and 738 ± 18 °C, respectively. Thus, these zircons were formed on the retrograde amphibolite-facies conditions at ∼8.0 kbar (M1-2). Titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite. They yielded a U–Pb age of ∼470 Ma and Zr-in-titanite temperature of 676 ± 23 °C at pressure of ∼7.0 kbar, suggesting that the amphibolite-facies retrogression perhaps persisted to ∼470 Ma.Weakly zoned zircons from the garnet amphibolite and inclusion-free titanites from the garnet pyroxenite gave consistent U–Pb ages of 418 ± 5 Ma and 423 ± 10 Ma, and Ti-in-zircon temperature of 742 ± 26 °C and Zr-in-titanite temperature of 764 ± 18 °C at ∼7.0 kbar, respectively. It is suggested that a heating event (M2) is registered by a subsequent phase of amphibolite-facies metamorphism. The ilmenite-bearing titanite crystals from the garnet pyroxenite yielded a U–Pb age of 352 ± 4 Ma, recording a late thermal event (M3).On the basis of combined petrological and geochronological results, we propose a revised tectonic model for the North Qinling orogeny in Palaeozoic. The high pressure granulites were formed by the northward subduction of the Shangdan oceanic slab and the arc-continent collision at ca. 500 Ma. Their exhumation happened at ca. 494–484 Ma as a result of slab breakoff. Subsequent amphibolite-facies metamorphism dated at ca. 440–420 Ma are coeval with the widespread magmatism in the North Qinling Terrane, which are likely caused by the reinitiation northward-subducted of Shangdan oceanic slab. At ca. 350 Ma, the North Qinling Terrane was likely affected by another thermal overprinting event. 相似文献
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秦岭松树沟岩体--一个遭受角闪岩相变质作用的超镁铁堆晶岩的实例 总被引:5,自引:0,他引:5
研究表明,松树沟岩体为一受到角闪岩相变质作用改造再结晶的层状超镁铁岩体,并保留着某些原岩面貌痕迹。其变质作用不仅导致微观上矿物变形和宏观上岩体的构造变形,而且也引起岩体内部矿物重新组合和再造,形成以含角闪石(直闪石和透闪石)为特征的Anth- Ol- En和Tre- Ol- Di两个变质矿物共生组合。其形成温度大约为70 0~75 0℃,压力为0 .6~0 .7GPa以下。岩相构造分析表明,该岩体并不是一个简单的透镜体。实际是由三个呈对称分布的岩相带组成的层状杂岩体,遭受同构造变形形成具有紧密褶曲特征的同斜向斜岩体。该岩体为一蛇绿岩构造岩块,其原岩来自大洋岩石圈板块壳层下部单元——由纯橄榄岩-透辉橄榄岩-透辉岩组成的层状超镁铁堆晶岩。它在遭受角闪岩相变质作用之前,曾经历过低变质蛇纹石化阶段(形成蛇纹岩)。恢复其原岩的原始层序具有垂直分异和多旋回岩浆演化的特点。故松树沟岩体不具有阿尔卑斯型橄榄岩,或地幔橄榄岩的成因属性。 相似文献
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东秦岭松树沟超镁铁质岩中辉石巨晶的成因和^40Ar—^39Ar定年及其地质意义 总被引:9,自引:0,他引:9
对东秦岭松树沟蛇绿岩残片中超镁铁质岩中的辉石巨晶化学成分的系统研究 ,表明其具有高压变质成因的特点 ;采用 4 0 Ar/ 39Ar快中子活化法得其高温坪年龄为 833.8± 4 Ma,等时线年龄为 84 8.2± 4 Ma。考虑到辉石对 Ar的封闭温度(6 2 0℃ )明显低于该岩片早期塑性变形变质温度 (≥ 80 0℃ ) ,因此该年龄不代表形成辉石巨晶的峰期高压变质时代 ,而代表了该岩体变质后初始抬升的冷却事件。这一年龄值与区内蛇绿岩中的镁铁质变质岩获得的变质年龄 (983± 14 0 Ma)在误差范围内大体一致 ,暗示松树沟蛇绿岩片可能整体经历了俯冲至下地壳深度遭受高压变质后再抬升的地质演化过程 相似文献
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东秦岭松树沟超镁铁质岩体地球化学和铂族元素特征: 对成因的指示 总被引:2,自引:0,他引:2
商南县松树沟超镁铁质岩体位于秦岭造山带商州—丹凤断裂以北,出露面积超过20 km2,是中国境内出露最大的阿尔卑斯型橄榄岩体,并以侵位方式与秦岭杂岩接触。该岩体主要由细粒纯橄榄岩、中粗粒纯橄榄岩和方辉橄榄岩等组成。地球化学特征显示明显亏损特征,同时不相容元素(从Rb到Nb)和LREE等元素相对HREE富集;纯橄榄岩的PGE含量明显低于方辉橄榄岩。与典型蛇绿岩的地幔橄榄岩对比研究,认为该岩体是经过熔体渗滤过程形成的纯橄榄岩体。 相似文献
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对北秦岭松树沟阿尔卑斯型超镁铁岩体接触变质带的角闪岩和斜长角闪片岩进行了全岩和单矿物氧同位素组成测定。矿物对氧同位素地质测温结果得到,角闪岩为745℃~770℃,角闪片岩为500℃~605℃,均与已知的岩石学温度计结果相吻合,指示了这些变质岩内部的氧同位素平衡。靠近超镁铁岩体的角闪岩受热触变质作用影响,导致了矿物之间的氧同位素再平衡;远离超镁铁岩的角闪片岩基本未受热触变质作用影响,因此氧同位素温度代表了秦岭群区域变质岩的形成温度。角闪岩和斜长角闪片岩全岩δ18O值为8.5~10.3,指示变质前原岩成分可能具有中基性火成岩特征 相似文献