“Grenvillian” intra-plate mafic magmatism in the southwestern Yangtze Block, SW China
“Grenvillian” intra-plate mafic magmatism in the southwestern Yangtze Block, SW China
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DOI：
10.1016/j.precamres.2013.12.019
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发表时间：
2014-03
影响因子：
3.8
通讯作者：
W. Chen;Weihua Sun;Wei Wang;J. Zhao;Mei‐Fu Zhou
中科院分区：
文献类型：
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作者：
W. Chen;Weihua Sun;Wei Wang;J. Zhao;Mei‐Fu Zhou
华南陆块格林威尔期岩浆活动的构造亲合性对于理解该陆块在中元古代Rodinia超大陆再造中的作用具有重要意义。在扬子地块西南部，中元古界末的巨林群普登组中，变质玄武岩呈层状产出。大多数变质玄武岩是高度变质的斜长角闪岩，主要由斜长石和角闪石组成，并含有少量的铁钛氧化物、钛铁矿和磷灰石。两个变质玄武岩样品的锆石U-Pb年龄均为1050 Ma。这些玄武岩的组成是拉斑玄武岩，具有高的和可变的TiO 2（1.1-3.2重量%）和低Mg#值（=Mg/（Mg + Fe 2+）; 35-60）。在原始地幔标准化图解上，它们以富集Th、Nb、Ta和轻稀土元素为特征，无明显的Zr-Hf-Nb-Ta-Ti异常。样品的Nd（t）值为−1.4 ~+4.0，锆石颗粒的Hf（t）值为−1.7 ~+11.0。低的Mg#值和Cr（55.1-261 ppm）和Ni（13.5-76.5 ppm）含量表明橄榄石和/或单斜辉石的分离结晶，而轻微的负Eu异常表明斜长石的轻微分离结晶。高的正Nd（t）和正Hf（t）值表明母岩浆来源于亏损的软流圈地幔源区。另一方面，变化的Nd（t）和Hf（t）值和La/Sm和Nb/La比值与该地区古元古代岩石的显著地壳同化作用相一致。这些样品具有高Ti和Ti/V比（大多数>40），典型的板内玄武岩。在构造判别的Zr-Nb-Y和Th-Ta-Hf图解中，地壳混染程度最低的样品被标绘在“板内”和“E-MORB”区域，表明其处于大陆内裂谷环境。鉴于新元古代弧岩浆活动（950-730 Ma）在该地区，我们认为，这些玄武岩可能产生在一个裂谷盆地的被动边缘。结果表明，扬子地块西南部的Grenville期岩浆活动不是造山作用的产物，从而反驳了传统上将华南地块置于Rodinia超大陆中部的观点。
Tectonic affinity of Grenvillian magmatism in the South China Block is important for understanding the role of the block in the reconstruction of the Rodinia supercontinent during the Mesoproterozoic. In the southwestern Yangtze Block, South China, late Mesoproterozoic metabasalts occur as layers in the Pudeng Formation of the Julin Group. Most metabasalts are highly metamorphosed to be amphibolites composed of dominantly plagioclase and amphibole with minor Fe–Ti oxides, titanite and apatite. Two metabasalt samples have identical zircon U–Pb ages of ∼1050 Ma. These basalts are tholeiitic in compositions and have high and variable TiO2(1.1–3.2 wt.%) and low Mg# values (=Mg/(Mg + Fe2+); 35–60). In the primitive mantle-normalized diagram, they are characterized by enrichments in Th, Nb, Ta and light REE without significant Zr–Hf–Nb–Ta–Ti anomalies. The samples haveɛNd(t) values ranging from −1.4 to +4.0 and their zircon grains haveɛHf(t) values from −1.7 to +11.0. The low Mg# values and Cr (55.1–261 ppm) and Ni (13.5–76.5 ppm) contents suggest fractional crystallization of olivine and/or clinopyroxene, whereas minor negative Eu anomalies indicates slightly fractional crystallization of plagioclase. High positiveɛNd(t) andɛHf(t) values suggest derivation of the parental magma from a depleted asthenospheric mantle source. On the other hand, variableɛNd(t) andɛHf(t) values and La/Sm and Nb/La ratios are consistent with significant crustal assimilation by the Paleoproterozoic rocks in the region. These samples have high Ti and Ti/V ratios (mostly >40), typical of within-plate basalts. In the Zr–Nb–Y and Th–Ta–Hf diagrams for tectonic discrimination, the samples with least crustal contamination are plotted in the ‘within-plate’ and ‘E-MORB’ fields, indicating an intra-continental rifting setting. In view of subsequent Neoproterozoic arc magmatic activities (950–730 Ma) in the region, we suggest that these basalts were likely produced at a rifting basin in a passive margin. Results presented here demonstrate that the Grenville-age magmatism in the southwestern Yangtze Block was not orogenic in origin, thus arguing against the existence of Grenvillian Orogen in the region that was traditionally used to place the South China Block in the central part of the Rodinia supercontinent.