南祁连大道尔吉早古生代弧后盆地型蛇绿岩的年代学、地球化学特征及意义
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摘要
大道尔吉蛇绿岩是中祁连地块和南祁连褶皱带之间蛇绿混杂岩带的重要组成部分。出露的岩石单元包括地幔橄榄岩、镁铁–超镁铁质堆晶杂岩和玄武安山岩。其中镁铁–超镁铁质堆晶杂岩包括三个堆晶旋回,单个堆晶旋回底部为含铬尖晶石纯橄岩,向上逐渐变为透辉石岩(辉石橄榄岩)→辉长岩等。堆晶杂岩中辉石橄榄岩Sm-Nd同位素等时线年龄为441±58 Ma,为加里东中期岩浆活动的产物。地球化学特征显示:地幔橄榄岩MgO(40.41%~40.96%)、Cr(3590×10~(–6)~7340×10~(–6))、Ni(1480×10~(–6)~1710×10~(–6))含量高,而Al_2O_3(0.35%~0.59%)、TiO_2(0.03%~0.04%)和∑REE(0.81×10~(–6)~1.84×10~(–6))含量低,具亏损地幔岩的特征;镁铁–超镁铁质堆晶杂岩的主量元素变化大,重稀土元素富集,正Eu异常(δEu=1.22~2.15);玄武安山岩SiO_2为54.90%~57.76%,MgO为3.50%~5.45%,TiO_2为0.72%~1.12%,Na_2O/K_2O>1,∑REE为24.9×10~(–6)~53.4×10~(–6),LaN/YbN=1.0~1.7,具平坦稀土配分模式,与E-MORB相似,同时富集Ba、Rb、Th等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素,具弧后盆地火山岩地球化学特征。综合区域地质、岩石组合和地球化学特征,认为大道尔吉蛇绿岩属构造肢解的蛇绿岩残片,具有俯冲带(SSZ)型蛇绿岩的特征,形成于弧后盆地环境,为奥陶纪柴北缘洋向中祁连地块俯冲,引起弧后扩张形成。
The Dadaoerji ophiolite is an important part of the ophiolitic mélange situated inbetween the Central Qilian block and the South Qilian fold belt. This ophiolite mainly consists of metaperidotite, mafic-ultramafic cumulate complexes and basaltic andesite. The mafic-ultramafic cumulate complex includes three cyclic cumulate. The bottom of each cyclic cumulate is chromium spinel-bearing dunite, and overlain by mafic and ultramafic laminated complex composed of pyroxene-peridotites, diopsidite, and gabbro. The Sm-Nd isochron age of the pyroxene-peridotites in the cumulate complex is 441±58 Ma, which indicates that the ophiolite was formed in Caledonian. Geochemical data show that the metamorphic peridotites have relatively higher Mg O contents of 40.41%~40.96%, Cr contents of 3590×10-6~ 7340×10~(–6), and Ni contents of 1480×10-6~1710×10-6, and lower Al_2O_3 contents of 0.35%~0.59%, Ti O2 contents of 0.03%~0.04%, and REE contents of 0.81×10-6~1.84×10~(–6), typical of depleted mantle rocks. The major elements of mafic-ultramafic cumulate complex show a wide range of variation, relatively enriched in HREE with positive Eu anomaly. The basaltic andesites have SiO_2 contents of 54.90%~57.76%, MgO contents of 3.50%~5.45%, TiO_2 contents of 0.72%~1.12%, Na_2O/K_2O>1; The total REE contents of the basaltic andesites are 24.9×10~(–6)~53.4×10~(–6), LaN/YbNratios are 1.0~1.7 with flat REE patterns similar to that of E-MORB. The volcanic rocks are enriched in LILE(Ba, Rb, Th) and depleted in HFSE(Nb, Ta, Zr, Ti), and are interpreted to be formed in a back-arc basin setting. Combined with the tectonic locations of the ophiolite and field observation, the authors propose that the Dadaoerji ophiolite is a relic slice of oceanic crust derived from depleted mantle, similar to the SSZ type ophiolite, and was formed in a back-arc basin setting. The North Qaidam oceanic plate was subducted northward under the Central Qilian blocks in the Early-Middle Ordovician.
The Dadaoerji ophiolite is an important part of the ophiolitic mélange situated inbetween the Central Qilian block and the South Qilian fold belt. This ophiolite mainly consists of metaperidotite, mafic-ultramafic cumulate complexes and basaltic andesite. The mafic-ultramafic cumulate complex includes three cyclic cumulate. The bottom of each cyclic cumulate is chromium spinel-bearing dunite, and overlain by mafic and ultramafic laminated complex composed of pyroxene-peridotites, diopsidite, and gabbro. The Sm-Nd isochron age of the pyroxene-peridotites in the cumulate complex is 441±58 Ma, which indicates that the ophiolite was formed in Caledonian. Geochemical data show that the metamorphic peridotites have relatively higher Mg O contents of 40.41%~40.96%, Cr contents of 3590×10-6~ 7340×10~(–6), and Ni contents of 1480×10-6~1710×10-6, and lower Al_2O_3 contents of 0.35%~0.59%, Ti O2 contents of 0.03%~0.04%, and REE contents of 0.81×10-6~1.84×10~(–6), typical of depleted mantle rocks. The major elements of mafic-ultramafic cumulate complex show a wide range of variation, relatively enriched in HREE with positive Eu anomaly. The basaltic andesites have SiO_2 contents of 54.90%~57.76%, MgO contents of 3.50%~5.45%, TiO_2 contents of 0.72%~1.12%, Na_2O/K_2O>1; The total REE contents of the basaltic andesites are 24.9×10~(–6)~53.4×10~(–6), LaN/YbNratios are 1.0~1.7 with flat REE patterns similar to that of E-MORB. The volcanic rocks are enriched in LILE(Ba, Rb, Th) and depleted in HFSE(Nb, Ta, Zr, Ti), and are interpreted to be formed in a back-arc basin setting. Combined with the tectonic locations of the ophiolite and field observation, the authors propose that the Dadaoerji ophiolite is a relic slice of oceanic crust derived from depleted mantle, similar to the SSZ type ophiolite, and was formed in a back-arc basin setting. The North Qaidam oceanic plate was subducted northward under the Central Qilian blocks in the Early-Middle Ordovician.
引文
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孟繁聪,张建新,郭春满,李金平.2010.大岔大坂MOR型和SZZ型蛇绿岩对北祁连洋演化的制约.岩石矿物学杂志,29(5):453-466.
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沈渭洲,高剑峰,徐士进,李惠民,周国庆,杨铸生,杨七文.2003.四川石棉蛇绿岩的地球化学特征及其构造意义.地质论评,49(1):17-27.
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吴才来,杨经绥,Ireland T,Wood J,李海兵,万渝生,史仁灯.2001.祁连南缘嗷唠山花岗岩SHRIMP锆石年龄及其地质意义.岩石学报,17(2):215-221.
吴才来,杨经绥,许志琴,Wood J,Ireland T,李海兵,史仁灯,孟繁聪,陈松永,Persing H,Meibom A.2004.柴达木盆地北缘古生代超高压带中花岗质岩浆作用.地质学报,78(5):658-674.
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许志琴,杨经绥,吴才来,李海兵,张建新,戚学祥,宋述光,万渝生,陈文,邱海峻.2003.柴达木北缘超高压变质带形成与折返的时限及机制.地质学报,77(2):163-176.
袁桂邦,王懋功,李惠民,郝国杰,辛后田,张宝华,王青海,田琪.2002.柴北缘绿梁山地区辉长岩的锆石U-Pb年龄及意义.前寒武纪研究进展,25(1):36-40.
赵虹,王崇礼.2004.甘肃南祁连党河南山中奥陶世火山岩的地球化学特征.现代地质,18(1):64-69.
张国英,丁书宏.2004.南祁连西段清水沟南巴龙贡嘎尔组地层特征及时代讨论.甘肃地质学报,13(1):38-45.
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