Petrogenesis and geodynamic settings of monzonitic granite at the end of the Neoarchean in Tianzhen-Huai'an area
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摘要: 通过详细的野外地质工作，本文在天镇-怀安地区怀安杂岩中识别出一套与TTG 片麻岩共生的二长花岗岩。LA-ICP-MS与SHRIMP锆石U-Pb定年分别获得二长花岗岩形成年龄为2 472±10 Ma、2 448±9 Ma。二长花岗岩样品SiO2含量70.55%～74.29%，Al2O3含量13.78%～15.35%，K2O含量4.26%～5.74%，Na2O含量3.04%～4.58%，K2O/Na2O除一件样品为0.93外，其余均大于1，在1.39～1.89之间，属钾质花岗岩；另外，样品CaO含量1.02%～1.37%，MgO含量0.15%～0.47%，TFeO 含量0.76%～1.9%，TiO2含量0.16%～0.32%，整体显示富Si、Na、K、Al，贫Ca、Fe、Mg、Ti 的特点。二长花岗岩轻稀土相对富集、重稀土亏损，微量元素富集Rb、Ba、K、Sr等大离子亲石元素，亏损U、Nb、Ta、P、Ti等高场强元素，同时Th含量较高而Cr、Ni含量低，显示与中、上部地壳非常相似的稀土、微量特征；而样品Sr含量相对较高，但Y、Yb含量很低，指示形成于中高压力条件下。综合上述认识，怀安杂岩中的二长花岗岩应是加厚下地壳部分熔融的产物，标志着华北克拉通北缘新太古代末期陆壳趋于成熟，已完成克拉通化。Abstract: Through detailed field geological work around Huaian-Tianzhen area, a suite of monzogranites has been distinguished from the TTG gneiss in the Huai’an complex.The zircon U-Pb ages of one monzogranite sample are 2 472±10 Ma and 2 448±9 Ma respectively obtained through LA-ICP-MS and SHRIMP methods.The contents of SiO2, Al2O3, K2O, Na2O and K2O are 70.55%~74.29%, 13.78%~15.35%, 4.26%~5.74% and 3.04%~4.58% respectively.The samples are attributed to potash granite with K2O/ Na2O=1.29~1.89, except for only one with K2O/ Na2O=0.93.The contents of Cao, MgO, TFeO and TiO2 of the monzogranite are 1.02%~1.37%, 0.15%~0.47%, 0.76%~1.9% and 0.16%~0.32% respectively, indicating enrichment in elements of Si, Na, K, Al, and depletion in elements of Ca, Fe, Mg, Ti.In addition, the monzogranite show relative enrichment in LREE(Rb, Ba, K and Sr)and depletion in HREE(U, Nb, Ta, P and Ti), with higher content of Th and lower content of Cr and Ni, indicating similar REE and trace elements characteristics with the middle-upper crust.The samples with higher content of Sr and lower content of Y and Yb are supposed to be formed in condition with middle to high pressure.Based on the above, the monzogranite in Huai'an complex should be product of partial melting of thickened lower crust, indicating the continental crust of the northern margin of North China Craton was mature and had already accomplished its cratonization in late Neoarchean.
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Key words:
- monzonitic granite /
- Huai’an complex /
- Neoarchean /
- North China Craton
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