Petrogenesis of the Baishan granite stock, Eastern Tianshan, NW China: Geodynamic setting and implications for potential mineralization
Petrogenesis of the Baishan granite stock, Eastern Tianshan, NW China: Geodynamic setting and implications for potential mineralization
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中国西北部东天山白山花岗岩群的岩石成因：地球动力学背景及其对潜在成矿的影响
DOI：
10.1016/j.lithos.2017.09.010
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发表时间：
2017-11
期刊：
影响因子：
3.5
通讯作者：
Gang Deng
中科院分区：
文献类型：
--
作者：
Mingjian Cao;Kezhang Qin;Guangming Li;Noreen J. Evans;Brent I.A. McInnes;Weiwei Lu;Gang Deng
白山花岗岩体位于铜、镍、钼矿化富集区，但因其原因至今仍是个谜。通过全岩元素分析、Sr-Nd同位素分析、多种矿物主量元素分析、锆石微量元素分析、U-Pb和Hf同位素分析，揭示了花岗岩的成因。所有花岗岩都显示出典型的I型特征，包括偏铝质到轻微过铝质、钙碱性特征，Nb、Ta、Ti和P强烈亏损，轻稀土元素和大离子亲石元素富集（例如，Cs、Rb、Th、U、K）。此外，Ti和P的强烈亏损、轻稀土元素高度分馏和重稀土元素分馏较少的模式，以及SiO2与TiO 2、Al 2 O3、MgO、FeOT、P2 O 5、Zr和Hf的负相关性，表明角闪石、磷灰石、锆石和含钛矿物具有明显的分馏结晶作用。全岩Sr-Nd和锆石Hf同位素组成变化较大，（87 Sr/86 Sr）i值为0.70358 ~ 0.70505，εNd（t）值为3.8 ~ 7.2，εHf（t）值为2.4 ~ 12.2，表明其来源于年轻下地壳的部分熔融，并有明显的古地壳加入。锆石U-Pb年龄显示成矿年龄为292 Ma，明显早于成矿花岗斑岩，略早于东天山区域镁铁质-超镁铁质、A型和闪长岩浆活动。花岗岩类很可能是在早二叠世地幔柱上方上升的软流圈地幔加热过程中形成的。矿物化学、饱和测温、矿物种类和全岩Fe 2 O3/FeO比表明，花岗岩原料的结晶温度> 980至665 °C，压力约为1.6 kbar，氧逸度≤ NNO。通过对早二叠世贫花岗岩和晚三叠世含矿花岗斑岩的地球化学、岩浆来源和结晶环境的对比，发现花岗岩中的低Sr/Y比值和低（La/Yb）N、氧化态还原（≤ NNO）是早二叠世贫花岗岩的标志。研究表明，在高Sr/Y、（La/Yb）N和高氧化环境下，花岗岩具有较高的钼成矿潜力。
Located in a region rich in Cu-Ni and Mo mineralization, the Baishan granitic stock is barren for reasons that remain enigmatic. Whole rock elemental and Sr-Nd isotope analysis, major element analysis of a number of minerals, and zircon trace element, U-Pb and Hf isotope analysis were undertaken in order to reveal the petrogenesis of the granites. All granites show typical I-type characteristics including metaluminous to slightly peraluminous, calc-alkaline signatures with a strong depletion of Nb, Ta, Ti and P, enrichment of light rare earth elements and large ion lithophile elements (e.g., Cs, Rb, Th, U, K). In addition, a strong depletion in Ti and P, highly fractionated light rare earth element patterns and less fractionated heavy rare earth element patterns, and negative correlations between SiO2and TiO2, Al2O3, MgO, FeOT, P2O5, Zr and Hf suggest significant fractional crystallization of amphibole, apatite, zircon and Ti-bearing minerals. Whole rock Sr-Nd and zircon Hf isotopic compositions show wide variations with (87Sr/86Sr)ivalues of 0.70358 to 0.70505, εNd (t) of 3.8 to 7.2, and εHf (t) of 2.4 to 12.2 indicating derivation from partial melting of juvenile lower crust with obvious addition of ancient crust. Zircon U-Pb ages indicate a formation age of ~ 292 Ma, significantly older than the ore-forming granite porphyry and slightly older than the regional mafic-ultramafic, A-type and diabase magmatism of Eastern Tianshan. The granite stocks were likely derived during heating of ascending asthenospheric mantle above a mantle plume in the Early Permian. Mineral chemistry, saturation thermometry, mineral species and whole rock Fe2O3/FeO ratios indicate a crystallization temperature of > 980 to 665 °C, pressure of ~ 1.6 kbar and oxygen fugacity of ≤ NNO for the granite stock. Comparing the geochemistry, magma source and crystallization environment for the Early Permian barren granite and Late Triassic ore-related granite porphyry, the low ratios of Sr/Y and low (La/Yb)N, and reduced oxidation state (≤ NNO) in the granitic stock are signatures of infertility for the Early Permian granite. This study implies high Mo mineralization potential for granitic rocks with high Sr/Y, (La/Yb)Nand highly oxidized conditions.
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