Petrogenesis of Early Paleocene Dengtong Volcanic-Plutonic Complex in Central Lhasa Terrane and Evolution of Crustal High-Silica Magma
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摘要: 花岗岩-流纹岩的成因研究是认识地壳演化机制的重要途径.通过岩石学、地球化学和同位素地质学方法，对拉萨地体中部措麦地区灯垌破火山机构的火山-侵入杂岩进行了成因研究.灯垌火山-侵入杂岩主要由花岗斑岩、流纹岩和流纹质熔结凝灰岩组成，锆石SHRIMP U-Pb年龄分别为64.1±0.8 Ma、62.9±0.7 Ma和63.2±0.7 Ma，形成时代一致.他们同属高钾钙碱性准铝质-过铝质岩浆岩，亏损高场强元素、富集大离子亲石元素，轻稀土元素富集、重稀土元素平坦，具有一致的锆石δ18O值（6.15‰~7.34‰），为同源岩浆演化的产物.流纹岩与花岗斑岩亏损Ba、Sr、P和Ti元素，具显著的负Eu异常，是岩浆发生不同程度分异演化的产物，前者代表晶粥体分离的熔体相，而后者是晶粥体富矿物相部分的产物.流纹质熔结凝灰岩轻重稀土元素分异程度相对较弱，具中等-弱的Eu负异常，是晶粥体自身被活化喷发的产物.结合前人研究成果，认为灯垌火山-侵入杂岩可能形成于古新世新特提斯洋北向俯冲于拉萨地体之下的过程中，俯冲带流体进入地幔楔并使其部分熔融，形成的幔源物质上涌，使地壳部分熔融形成的中酸性岩浆侵入或喷发而形成侵入岩或火山岩.Abstract: Genetic study on the granite-rhyolite associations is an essential way to uncover the mechanism of crustal evoltion. Geochemistry, zircon U-Pb dating, and oxygen isotopes of the Dengtong volcanic-plutonic complex in the central Lhasa Terrane are reported in this paper. Zircon SHRIMP U-Pb ages for the granite porphyry, rhyolite and rhyolitic ignimbrite samples are 64.1±0.8 Ma, 62.9±0.7 Ma and 63.2±0.7 Ma, respectively. These rocks display similar geochemical characterstics. They are high K calc-alkaline and weakly peraluminous, exhibit HFSE depletion, LILE and LREE enrichment, and flat HREE, with consistent zircon oxygen isotopic composition (δ18O=6.15‰-7.34‰), suggesting consecutive magma evolution of cognate origin. The rhyolite and granite porphyry are depleted in Ba, Sr, P and Ti with significant negative Eu anomaly, indicating conspicuous fractionation. The rhyolite presents the extracted melt from the mush enriched in mineral phases, while the granite porphyry generated from the mush by fractional crystallization. The rhyolitic ignimbrite displays relatively weak negative Eu anomaly, and was generated by the eruption of the residual magma mush. Considering the temporal and spatial distribution of the Paleocene magmatic rocks along the central Lhasa Terrane, it is proposed that the roll-back of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere subducted beneath the Lhasa Terrane was responsible for the geodynamic regime of the Dengtong volcanic-plutonic complex. The upwelling of the asthenosphere material caused partial melting of lower crust, and resulted in intrusive or volcanic rocks.
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Key words:
- magma evolution /
- volcanic-plutonic complex /
- zircon U-Pb-O isotopes /
- Lhasa Terrane /
- geochronology /
- geochemistry
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图 1 青藏高原构造单元划分(a)据和拉萨地体林子宗群火山岩分布(b)
图a据Zhang and Zhang (2017)；图b据莫宣学等(2003)修改，BNS.班公湖-怒江缝合带，YZS.雅鲁藏布江缝合带
Fig. 1. Tectonic framework of the Tibet Plateau (a) and distribution of the Linzizong Group volcanic rocks at the Lhasa Terrane(b)
图 2 灯垌破火山机构地质简图
据中华人民共和国1∶25万区域地质调查报告措麦区幅，地质出版社，2002
Fig. 2. Simplified geological map of the Dengtong caldera
图 3 灯垌火山-侵入杂岩的主要岩石类型和显微照片
Pl.斜长石; Kfs.钾长石；Q.石英；Bt.黑云母
Fig. 3. The main rock types and microphotographs of the Dengtong volcanic-plutonic complex
图 5 锆石δ18O值频数分布
幔源锆石δ18O平均值据Valley et al.(1998)；正常岩浆锆石δ18O值据Blum et al.(2016)
Fig. 5. Histograms of zircon δ18O values
图 6 地球化学分类图解
图a据Irvine and Baragar (1971)；图b据Peccerillo and Taylor (1976)；图c据Maniar and Piccoli (1989)；图d据Frost et al. (2001)
Fig. 6. Geochemical classification diagrams
图 7 稀土元素球粒陨石标准化图解(a、c)和微量元素原始地幔标准化图解(b、d) (标准化值据Sun and McDonough, 1989)
Fig. 7. Chondrite-normalized REE patterns (a, c) and primitive mantle-normalized trace elements spider diagrams (b, d) (normalized values after Sun and McDonough, 1989)
图 8 灯垌火山-侵入岩的元素相关图解
Fig. 8. Elemental co-variation diagrams of the Dengtong volcanic-plutonic rocks
图 9 灯垌火山-侵入岩稀土、微量元素的相关图解
Fig. 9. Diagrams of REE and trace elements for the Dengtong volcanic-plutonic rocks
图 10 构造判别图解(据Pearce et al., 1984)
Fig. 10. Tectonic discrimination diagrams (after Pearce et al., 1984)
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