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21.
该工作在藏南日喀则弧前盆地砂岩中发现了大量碎屑铬尖晶石。电子探针化学成分分析显示这些铬尖晶石具有高铬(铬指数Cr#为0.52~0.89)、低Fe3+含量(Fe3+/ Fe2+<0.5)、低TiO2含量(多小于0.2%)的特征,指示这些铬尖晶石来源于与洋壳俯冲作用有关的橄榄岩和火成岩,因此弧火山岩和SSZ型蛇绿岩套是其最可能的物源。考虑到日喀则弧前盆地的碎屑物主要来源于拉萨地体,而拉萨地体北侧的班公-怒江缝合带的碎屑物质不可能穿过晚白垩世时期已经隆起的冈底斯岩浆弧。笔者推测,铬尖晶石可能来自于冈底斯弧和拉萨地体内部已经消失的古蛇绿岩套。 相似文献
22.
OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS 相似文献
23.
Platinum-group elemental geochemistry of mafic and ultramafic rocks from the Xigaze ophiolite, southern Tibet 总被引：4，自引：0，他引：4
The Xigaze ophiolite in the central part of the Yarlung–Zangbo suture zone, southern Tibet, has a well-preserved sequence of sheeted dykes, basalts, cumulates and mantle peridotites at Jiding and Luqu. Both the basalts and diabases at Jiding have similar compositions with SiO2 ranging from 45.9 to 53.5 wt%, MgO from 3.1 to 6.8 wt% and TiO2 from 0.87 to 1.21 wt%. Their Mg#s [100Mg/(Mg + Fe)] range from 40 to 60, indicating crystallization from relatively evolved magmas. They have LREE-depleted, chondrite-normalized REE diagrams, suggesting a depleted mantle source. These basaltic rocks have slightly negative Nb- and Ti-anomalies, suggesting that the Xigaze ophiolite represents a fragment of mature MORB lithosphere modified in a suprasubduction zone environment. The mantle peridotites at Luqu are high depleted with low CaO (0.3–1.2 wt%) and Al2O3 (0.04–0.42 wt%). They display V-shaped, chondrite-normalized REE patterns with (La/Gd)N ratios ranging from 3.17 to 64.6 and (Gd/Yb)N from 0.02 to 0.20, features reflecting secondary metasomatism by melts derived from the underlying subducted slab. Thus, the geochemistry of both the basaltic rocks and mantle peridotites suggests that the Xigaze ophiolite formed in a suprasubduction zone.Both the diabases and basalts have Pd/Ir ratios ranging from 7 to 77, similar to MORB. However, they have very low PGE abundances, closely approximating the predicted concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt, indicating that the rocks originated from magmas that were S-saturated before eruption. Moderate degrees of partial melting and early precipitation of PGE alloys explain their high Pd/Ir ratios and negative Pt-anomalies. The mantle peridotites contain variable amounts of Pd (5.99–13.5 ppb) and Pt (7.92–20.5 ppb), and have a relatively narrow range of Ir (3.47–5.01 ppb). In the mantle-normalized Ni, PGE, Au and Cu diagram, they are relatively rich in Pd and depleted in Cu. There is a positive correlation between CaO and Pd. The Pd enrichment is possibly due to secondary enrichment by metasomatism. Al2O3 and Hf do not correlate with Ir, but show positive variations with Pt, Pd and Au, indicating that some noble metals can be enriched by metasomatic fluids or melts carrying a little Al and Hf. We propose a model in which the low PGE contents and high Pd/Ir ratios of the basaltic rocks reflect precipitation of sulfides and moderate degrees of partial melting. The high Pd mantle peridotites of Xigaze ophiolites were formed by secondary metasomatism by a boninitic melt above a subduction zone. 相似文献
24.
日喀则群砂泥质复理石的浊积层序非常发育,包括富泥的粘性高密度浊流、砂质高密度浊流、低密度浊流和Pickering及Hiscott(1986)的限制性泥质高密度浊流的沉积层序。其中,低密度浊流沉积层序的组合与鲍马(Bouma)层序很相似,两者的区别在于沉积的粒度、粒级分布范围、层序厚度和相组合等特征的不同。沉积相序、古流向和物源分析表明,日喀则群复理石盆地的南侧主要发育单向物源的海底扇体系,而盆地北侧以发育双向物源和多种沉积流体的复合沉积为特征。值得指出的是,特殊的浊积类型——限制性泥质高密度浊流沉积的发现为论证日喀则群复理石盆地属残留盆地又提供了一条有力的依据。 相似文献
25.
西藏仲巴县穷果北部日喀则弧前盆地地层再认识 总被引：2，自引：0，他引：2
通过对区内发育完好并保留下来的白垩纪—始新世地层的研究，海相白垩纪日喀则群自下而上可划分为昂仁组、帕达那组、曲贝亚组；古新世—始新世错江顶群包括下部曲下组和上部加拉孜组，整合覆于错江顶群之上的“如角藏布砂砾岩”为弧前盆地封盆粗碎屑岩。日喀则群与错江顶群在沉积学、地层古生物学及构造地质学诸方面的差异揭示了两群之间，更确切地说，曲贝亚组与曲下组之间呈不整合接触，该不整合形成于坎潘期—马斯特里赫特期与丹尼期之间。该区日喀则弧前盆地完整的地层序列全面再现了印度板块向欧亚板块俯冲过程，两板块的碰撞和新特提斯洋的彻底关闭发生在弧前盆地封盆粗碎屑岩沉积结束以后，这比原来认识的要晚些。 相似文献
26.
测定了日喀则地区蛇绿岩组合岩石的稀有气体同位素组成。白朗的玄武岩具有比较均一的He同位素组成 ,3He/ 4He平均值为 5 35 9Ra;变橄岩的He同位素比值范围为 1 10 4～ 3 384Ra,平均为 2 383Ra;从东部的仁布和大竹到西部的下鲁和白朗 ,辉绿岩的3He/ 4He由低变高。采于吉定的新鲜辉绿岩平均高达 31 5 7Ra,该值接近于在夏威夷发现的高比值。分步加热法结果显示高R值的He是在低温步释放的 ,根据这种高R值的He可以推测雅鲁藏布江蛇绿岩的成因环境有地幔柱的作用 ,该构造带很可能出现过裂谷洋盆或未能发育成熟的深大裂谷带 相似文献
27.
Structural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet was a typical slow-spreading ridge. A new field, geochemical, mineral, and U-Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow-spreading ridge. Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting. U-Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes. Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low-pressure (2–3 kbar) fractional crystallization of mid-ocean ridge basalt-like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks. The complex cross-cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion. The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow-spreading ridges, time-dependent on structural (external) and magmatic (internal) processes. 相似文献
28.
通过对日喀则蛇绿岩的镁铁质岩石进行岩石学和地球化学研究,探讨其岩石成因及源区性质,同时根据Re-Os同位素的地球化学约束来探索雅鲁藏布蛇绿岩的形成机制。研究表明,日喀则蛇绿岩镁铁质岩石微量元素的标准化配分型式与洋中脊玄武岩类似,又具有岛弧玄武岩的地球化学特征。结合微量元素和同位素特征,均表明岩石的形成与俯冲作用有关。日喀则蛇绿岩产出于远离大陆地壳的SSZ环境,其形成过程未受到陆壳的混染;同时,Re-Os同位素体系受蚀变作用的影响也不明显。日喀则蛇绿岩镁铁质岩石的Re、Os含量低,187Os/188Os同位素比值较高,主要是源区性质和俯冲作用影响的结果。特提斯洋早期发生的多次俯冲作用造成地幔源区不均一。新特提斯洋壳俯冲过程中,上述不均一地幔发生部分熔融产生的镁铁质岩浆上升,经过遭受了早期熔体/岩石作用的纯橄岩通道,发生强烈的Re-Os同位素分馏,使熔体与地幔残余Os同位素组成表现出明显的解耦现象,进而形成现今的日喀则蛇绿岩。 相似文献
29.
Xi-chen Wang Wei-liang Liu Xi-chong Hu Bin Xia Wei Huang 《International Geology Review》2018,60(10):1267-1289
The Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U–Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174–149 Ma and 137–123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive εNd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle–Late Jurassic ophiolitic massifs (174–149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150–137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137–123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle. 相似文献