Mineral Assemblages and Their Evolution of Clinopyroxenite Xenolith in Early Jurassic Intrusive Complex from Xu-Huai Region, Central China
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摘要: 对徐—淮地区早侏罗世侵入杂岩中尖晶石单斜辉石岩捕虏体的岩石学和矿物学研究表明, 镁铁尖晶石和富铝单斜辉石矿物组合代表了更早期地幔岩浆在下部地壳或上地幔顶部的堆积体, 其形成的温压条件为0.90 GPa和835℃; 以尖晶石转变成石榴石和单斜辉石+石榴石+刚玉组合为代表的矿物组合(Ⅱ) 反映了尖晶石单斜辉石岩形成后又经历了高压变质作用, 所估算的温压条件为1.40~1.6 5 GPa和900~1 000℃; 晚期退变角闪石的形成表明尖晶石单斜辉石岩又遭受了角闪岩相的退化变质作用, 估算的温压条件为0.70 GPa和670~740℃.该类捕虏体的发现对认识大别—苏鲁超高压变质带的深部俯冲方向及其范围具有重要意义.Abstract: Petrology and mineralogy of spinel-clinopyroxenite xenolith in Early Jurassic intrusive complex from Xu-Huai region, central China indicate that Mg-Fe spinel and Al-clinopyroxene assemblage could be present as the cumulate of the underplating magma in the lower crust or the most upper mantle, which were formed under 0.90 GPa and 835 ℃; that the mineral assemblage (Ⅱ) represented by spinel-garnet transition and clinopyroxene (Cpx) +garnet (Grt) +corundum (Co) assemblage could reflect that the spinel-clinopyroxenite was subjected to high pressure metamorphism with 1.40-1.65 GPa and 900-1 000 ℃; and that the occurrence of retrogressive amphibole implies that the spinel-clinopyroxenite could undergo the amphibolite facies retrogressive metamorphism with 0.70 GPa and 670-740 ℃. The discovery of the xenolith is of important implication for us to recognize the deep subduction direction and the scale of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt.
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图 1 早侏罗世侵入杂岩和大别—苏鲁超高压变质带分布
1.高温(超) 高压变质带; 2.低温高压变质带; 3.秦岭—桐柏深变质杂岩; 4.北淮阳浅变质杂岩; 5.古生界; 6.断层; 7.岩体; 8.断层
Fig. 1. Distribution of Early Jurassic intrusive complex and Dabie-Sulu UHP metamorphic belt
图 2 徐—淮地区地质略图
1.中生代和更年轻的地层; 2.晚古生代地层; 3.晚元古和早古生代地层; 4.逆冲断层; 5.正断层; 6.其他断层; 7.地层界线; 8.侵入体
Fig. 2. Geologic sketch map in Xu-Huai region
图 3 捕虏体产状和矿物组合的图片
a.单斜辉石岩的野外照片; b.捕虏体中尖晶石-石榴石转变的背散射图像; c.捕虏体中的尖晶石、刚玉和石榴石组合.Grt.石榴石; Sp.尖晶石; Co.刚玉; Cpx.单斜辉石
Fig. 3. Photomicrographs showing mineral microstructures
表 1 单斜辉石岩中的矿物成分
Table 1. Mineral composition in clinopyroxenite xenolith
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