Petrogenesis of Lincang granites in Sanjiang area of western Yunnan Province:Constraints from geochemistry, zircon U-Pb geochronology and Hf isotope
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摘要:
位于滇西三江地区南澜沧江带的临沧花岗岩，其岩石类型主要为黑云母二长花岗岩，研究结果表明，该花岗岩的SiO2含量为66.84%~73.99%，平均为69.72%，K2O/Na2O值高，为1.42~30.1，平均为8.66，Al2O3含量为12.94%~15.23%，平均为14.44%，铝饱和指数A/CNK为1.06~8.59，平均为2.61，大部分大于1.1，为高钾钙碱性过铝-强过铝花岗岩。岩石总体上富集大离子亲石元素和Pb，明显亏损高场强元素。稀土总量198.2×10-6~359.2×10-6，平均为252.5×10-6，具有明显的轻稀土富集，重稀土亏损的特征，(La/Yb)N为7.87~17.62，平均11.19，δEu为0.34~0.57，平均0.48，球粒陨石标准化配分模式显示明显的负Eu异常。两件样品的锆石U-Pb年龄分别为219.19±0.99Ma和219.69±0.67Ma，属晚三叠世。SiO2-P2O5、SiO2-Zr判别图、K2O-Na2O判别图、ACF图解等花岗岩成因类型判别图指示临沧花岗岩为S型花岗岩，其物质来源为贫粘土的砂屑岩。微量元素Rb-Y+Nd判别图中，临沧花岗岩体投影点全部落入后碰撞花岗岩区。在Sr-Yb判别图中，投影点大部分落入低Sr高Yb型花岗岩区，与我国东南沿海花岗岩特征一致，应形成于挤压向伸展转换的后碰撞阶段。锆石Hf同位素组成比较均一，εHf(t) 均为负值(集中于-14~-11之间)，Hf地壳模式年龄集中于1.95~2.15Ga，推断其为古老地壳部分熔融的产物。结合锆石定年结果及岩体产出的区域地质背景，我们认为临沧花岗岩形成于缅泰马陆块与思茅地块大陆碰撞造山过程的后碰撞阶段，应形成于晚三叠世。
Abstract:The Lincang granites in southern Lancangjiang zone of Sanjiang area, located in the west of Yunnan Province, are mainly composed of biotitic monzogranites. The SiO2 and Al2O3 of the granites range 66.84%~73.99% and 12.94%~15.23%, averagely 69.72% and 14.44% respectively. The K2O/Na2O ratio is high, ranging 1.42~30.1, with average of 8.66. A/CNK varies from 1.06 to 8.59, averagely 2.61, and mostly > 1.1. So Lincang granites are high-K calc-alkaline and peraluminous to strongly peraluminous series. Lincang granites are also enriched in LILEs and Pb, but strongly depleted in HFSEs. The REE content is between 198.2×10-6 and 359.2×10-6(average 252.5×10-6), with (La/Yb)N ratios of 7.87~17.62 (average 11.19) and δEu of 0.34~0.57 (average 0.48). The intrusion is remarkably characterized by enriched LREE and depleted HREE. The chondrite-normalized REE patterns show strong negative Eu anomalies. The zircon U-Pb ages of the two samples are 219.19±0.99Ma and 219.69±0.67Ma, belonging to Late Triassic. The SiO2-P2O5, SiO2-Zr, K2O-Na2O relationship and ACF plot indicate that the Lincang granites are S type granites, which mainly derive from clay-poor psammite. Lincang granites can be dropped into the group of post-collisional granites in the Rb-Y+Nd discrimination diagram. In addition, Lincang granites have the features of low-Sr and high-Yb in the Sr-Yb discrimination diagram, which is in accordance with the granites in southeastern China. Lincang granites have homogeneous zircon Hf isotopic composition. Their zircons have negative εHf(t) values (-14~-11) and old Hf isotope crust model ages (1.95~2.15Ga), suggesting that Lincang granites were formed by partial melting of old crust. Therefore, we consider that Lincang granites were formed in post-collisional stage between the Burma-Thai-Malaysia and Simao block in the Late Triassic.
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图 1
三江地区临沧花岗岩的地质简图(据云南省地质矿产局，1990修改)
Figure 1.
Simplified geological map of Lincang granites in Sanjiang area (modified after BGMRYP, 1990)
图 2
临沧花岗岩的野外露头及镜下特征
Figure 2.
Outcrop photograph and photomicrographs of Lincang granites
图 3
临沧花岗岩的K2O-SiO2图解(a，实线据Peccerillo and Taylor, 1976；虚线据Middlemost, 1985) 及A/CNK-A/NK图解(b，据Mania and Piccoli, 1989)
Figure 3.
K2O-SiO2(a, solid line after Peccerillo and Taylor, 1976; dash line after Middlemost, 1985) and A/CNK-A/NK (b, after Mania and Piccoli, 1989) plots for the Lincang granites
图 4
临沧花岗岩的QAP图解(据Streckeisen, 1976)
Figure 4.
QAP classification diagram for the Lincang granites (after Streckeisen, 1976)
图 5
临沧花岗岩的微量元素原始地幔标准化蛛网图及稀土元素球粒陨石标准化配分曲线图(标准化数值据Sun and McDonough, 1989)
Figure 5.
Primitive mantle-normalized trace element patterns (a) and chondrite-normalized REE patterns (b) for the Lincang granites (normalized data after Sun and McDonough, 1989)
图 6
临沧花岗岩代表性锆石的阴极发光图像(a, b) 和锆石U-Pb年龄谐和图(c, d)
Figure 6.
Cathodoluminescence (CL) images (a, b) of representative zircons and U-Pb zircon concordia plots (c, d) for the Lincang granites
图 7
临沧花岗岩体εHf(t) 值(a) 和Hf同位素地壳模式年龄(tDMC) 柱状图(b)
Figure 7.
Histograms of εHf(t) values (a) and Hf-isotope crust model ages (b) of the Lincang granites
图 8
临沧花岗岩体JH1024(a) 和LC1055(b)εHf(t)-t图解
Figure 8.
εHf(t)-t plot for JH1024(a) and LC1055(b) of the Lincang granites
图 9
临沧花岗岩Rb/Ba-Rb/Sr (a) 和CaO/Na2O-Al2O3/TiO2(b) 图解(据Sylvester, 1998)
Figure 9.
Rb/Ba-Rb/Sr (a) and CaO/Na2O-Al2O3/TiO2(b) diagrams of Lincang granites (after Sylvester, 1998)
图 10
临沧花岗岩岩石成因类型判别图
Figure 10.
Discrimination diagrams of genesis type for the Lincang granites
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