Abstract

A significant occurrence of olivine and serpentine (antigorite, lizardite) are reported for the first time in the Dihing sedimentary unit of upper Assam. Petrography of the Dihing Formation documents olivine, serpentine (antigorite, lizardite), pyroxene and major framework grains (quartz, feldspar and variety of lithic fragments). Based on modal composition, the Dihing unit samples are classified as lithic arenite. Heavy mineral analysis in addition to the above, provides evidence of other minerals such as zircon, tourmaline, rutile, garnet, epidote, amphibole, sphene, aluminosilicates, apatite, spinel, staurolite, chloritoid, chlorite and mica. The heavy mineral assemblage is dominated by olivine (36.9%) and pyroxene (16.5%). The low (10.7%) zircon–tourmaline–rutile (ZTR) index infers mineralogically immature nature of sediments. Raman spectroscopy confirms the presence of antigorite and electron microprobe analysis corroborates the existence of olivine, pyroxene, serpentine and chrome-spinel in the analyzed samples. The imbricated clasts and cross-bedding sedimentary structures indicate SSW to SW paleo-current direction. The peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are suggested as the likely sediment provenance for olivine and serpentine minerals. We advocate that the Lohit and Dibang rivers earlier flowed close to the Indo-Burman Ranges (IBR) depositing the Plio-Pleistocene Dihing sediments before merging with the Brahmaputra river.

Research Highlights

• First report of detrital olivine and serpentine minerals in the Dihing unit of upper Assam.

• Petrography, heavy mineral, XRD and EPMA analyses show the presence of olivine, pyroxene, serpentine and chrome spinel.

• Clast imbrications and current-bedding indicate SSW to SW paleocurrent direction.

• Peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are inferred as the source.

• The Lohit and Dibang rivers seem to have carried detritus from source to sink.

中文翻译：

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阿萨姆邦上游Dihing单元中存在碎屑橄榄石和蛇纹石矿物：对来源的启示

摘要

据报道，在阿萨姆邦上部的迪兴沉积单元中，橄榄石和蛇纹石（蛇纹石，蜥蜴石）大量出现。迪兴组的岩相学记录了橄榄石，蛇纹石（蛇纹石，蜥蜴石），辉石和主要骨架颗粒（石英，长石和各种碎屑岩）。根据模态成分，将Dihing单元样品归类为石质珍珠岩。除上述物质外，重矿物分析还提供了其他矿物的证据，例如锆石，电气石，金红石，石榴石，山茱epi，闪石，斜方石，铝硅酸盐，磷灰石，尖晶石，星形石，类滑石，亚氯酸盐和云母。重矿物组合主要是橄榄石（36.9％）和辉石（16.5％）。锆石-电气石-金红石（ZTR）指数低（10.7％）可以推断出沉积物的矿物学上不成熟的性质。拉曼光谱法证实了蛇纹石的存在，电子探针分析证实了所分析样品中橄榄石，辉石，蛇纹石和铬-尖晶石的存在。层状碎屑和交叉层理的沉积结构指示了南海到南海的古流向。来自Lohit–Dibang山谷的Tidding缝合带的橄榄岩和蛇纹岩岩石被建议作为橄榄石和蛇纹石矿物的沉积物来源。我们主张Lohit和Dibang河流在与Brahmaputra河合并之前，先流向印度－布尔曼山脉（IBR）沉积Plio-更新世Dihing沉积物。层状碎屑和交叉层理的沉积结构指示了南海到南海的古流向。来自Lohit–Dibang山谷的Tidding缝合带的橄榄岩和蛇纹岩岩石被建议作为橄榄石和蛇纹石矿物的沉积物来源。我们主张Lohit和Dibang河流在与Brahmaputra河合并之前，先流向印度－布尔曼山脉（IBR）沉积Plio-更新世Dihing沉积物。层状碎屑和交叉层理的沉积结构指示了南海到南海的古流向。来自Lohit–Dibang山谷的Tidding缝合带的橄榄岩和蛇纹岩岩石被建议作为橄榄石和蛇纹石矿物的沉积物来源。我们主张Lohit和Dibang河流在与Brahmaputra河合并之前，先流向印度－布尔曼山脉（IBR）沉积Plio-更新世Dihing沉积物。

研究重点

• 上阿萨姆邦迪兴单元的碎屑橄榄石和蛇纹石矿物的首次报道。

• 岩石学，重矿物，XRD和EPMA分析表明存在橄榄石，辉石，蛇纹石和铬尖晶石。

• 碎裂震荡和层流指示了SSW到SW的古电流方向。

• 推断出Lohit–Dibang山谷的Tidding缝合带的橄榄岩和蛇纹岩岩石为来源。

• Lohit河和Dibang河似乎将碎屑从源头带到下沉。