Studies on geochemistry, zircon U-Pb geochronology and Hf isotopes of granite in Wangfeng area at the northern margin of Middle Tianshan, Xinjiang
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摘要:
新疆中天山北缘望峰地区花岗岩的岩石学、地球化学及锆石U-Pb-Hf同位素研究表明：(1) 望峰地区花岗岩岩石类型主要为二云母二长花岗岩和黑云母二长花岗岩，岩体高SiO2、Al2O3，中等富碱且相对富钾，A/CNK≥1.1，稀土含量中等 (ΣREE=162.6×10-6~211.8×10-6)，轻重稀土分馏明显且富集轻稀土，具中等Eu负异常 (δEu=0.42~0.49)，相对富集LILE和LREE，亏损Ba、Sr、P、Ti、Nb、Ta、HREE等，属高钾钙碱性过铝质S型花岗岩；(2) 锆石LA-ICP-MS U-Pb定年获得岩浆结晶年龄为439.9±2.2Ma，成岩时代为早志留世；(3) 锆石Hf同位素组成较均一，εHf(t)=-5.0~-1.3，平均-2.72，二阶段模式年龄tDM2变化范围在1294~1482Ma之间，岩浆主要来源于中元古代中期地壳结晶基底岩系的部分熔融，源岩为经历过风化旋回、成熟度较低的硬砂岩，形成于中天山北缘早古生代碰撞造山期同碰撞环境。
Abstract:Rock samples in Wangfeng area consisting of two-mica monzonitic granite and biotite monzonitic granite locate in the northern margin of Middle Tianshan in Xinjiang, exhibiting as the direct gold-bearing country rocks, which synthetically display characteristics of peraluminous S-type granite or MPG belonging to high-K calcalkaline series: Allitic mineral assemblages of granitic rocks with medium-fine grained granitic texture and weakly gneissic structure comprise primary muscovite and biotite.Geochemically, Wangfeng granite is characterized by high SiO2, Al2O3 and ALK with K2O/Na2O ratios changing from 1.07 to 1.25 and 1.15 on average, A/CNK ratios range from 1.10 to 1.12 and 1.11 on average.Granite samples are rich in LREE and LILE (K, Rb, Th, U etc.), infertile in HREE and Ba, Sr, P, Ti, Nb, Ta etc., coupling with moderate Eu negative anomaly (δEu=0.42~0.49), which are in relation to magmatic fractionation.Wangfeng granite intruded at 439.9±2.2Ma based on the dating results of zircon U-Pb geochronology.Zircon Lu-Hf isotopic compositions indicate that εHf(t)=-5.0~-1.3 with an average of -2.72, two-stage model ages (tDM2) range from 1294Ma to1482Ma, which yields Wangfeng granite derived from partial melting of Mid-Mesoproterozoic crystallized basements formed during the process of metamorphism of greywacke combining with the plotting results of experimental petrology.In terms of both previous research results on regional tectonic setting and tectonic discrimination diagrams, Wangfeng granite formed at the syn-collisional stage of Caledonian Orogeny, there is a quite significant conclusion that Middle Tianshan may be compound, which is composed of orogenic belts with distinct inner frameworks.
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Key words:
- S-type granite /
- Early Silurian /
- Source rock /
- Syn-collisional orogeny /
- Wangfeng, Xinjiang
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图 1
新疆望峰地区地质简图 (据王居里，2011①，有修改；构造单元划分据Gao et al., 1998)
Figure 1.
Geological sketch map of Wangfeng area in Xinjiang (tectonic division modified after Gao et al., 1998)
图 2
望峰地区花岗岩显微镜下特征
Figure 2.
Micro-fabrics of granite in Wangfeng area under polarizing microscope
图 3
望峰地区花岗岩K2O-SiO2图 (a，实线据Peccerillo and Taylor, 1976，虚线据Middlemost，1985) 和A/NK-A/CNK图 (b，据Maniar and Piccoli, 1989)
Figure 3.
K2O-SiO2 diagram (a, solid line is after Peccerillo et al., 1976 and dashed line after Middlemost, 1985) and A/NK-A/CNK diagram (b, after Maniar and Piccoli, 1989) of granite in Wangfeng area
图 4
望峰地区花岗岩球粒陨石标准化稀土元素配分图 (a) 及微量元素原始地幔标准化蛛网图 (b)(标准化值据Sun and McDonough, 1989)
Figure 4.
Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) of granite in Wangfeng area (normalized values after Sun and McDonough, 1989)
图 5
望峰地区花岗岩锆石CL图像、测点位置及谐和年龄
Figure 5.
Zircon CL graphics, points sites and concordia ages of granite in Wangfeng area
图 6
望峰地区花岗岩锆石U-Pb定年结果
Figure 6.
Zircon U-Pb dating results of granite in Wangfeng area
图 7
望峰地区花岗岩源区物质组成的Rb/Ba-Rb/Sr图解 (a，源岩范围据Sylveste，1998) 和A/FM-C/FM图解 (b，Gerdes et al., 2000)
Figure 7.
Rb/Ba-Rb/Sr diagram(a，after Sylveste，1998)and A/FM-C/FM diagram(b，after Gerdes et al.，2000)of granite in Wangfeng area
图 8
望峰地区花岗岩Rb/10-Hf-3Ta (左，据Harris et al., 1986) 和R1-R2(右，据Bachelor and Bowden, 1985) 判别图解
Figure 8.
Rb/10-Hf-3Ta (left, after Harris et al., 1986) and R1-R2(right, after Bachelor and Bowden, 1985) tectonic discrimination diagrams of granite in Wangfeng area
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