Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydro...Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydrous uvarovite garnet having Cr_(2)O_(3) upto 19.66 wt% and CaO of 32.12–35.14 wt% in the serpentinized mantle peridotites of Naga Hills Ophiolite(NHO), India. They occur in association with low-Cr diopsides. They are enriched in LILE(Ba, Sr), LREEs, with fractionating LREE-MREE [avg.(La/Sm)_(N) = 2.16] with flat MREE/HREE patterns [avg.(Sm/Yb)_(N) = 0.95]. Raman spectra indicate the presence of hydroxyl(OH^(–)) peaks from 3500 to 3700 cm^(-1). Relative abundances in fluid mobile elements and their close association with clinopyroxenes are suggestive of the formation of uvarovite garnets through low temperature metasomatic alteration of low-Cr diopsides by hydrothermal slab fluids. The high LREE concentration and absence of Eu anomaly in the garnet further attest to alkaline nature of the transporting slab dehydrated fluid rather the involvement of low-p H solution. The chemical characteristics of the hydroxyl bearing uvarovite hosted by the mantle peridotite of NHO deviate from the classical features of uvarovite garnet, and their origin is attributed to the fluid-induced metasomatism of the sub arc mantle wedge in a suprasubduction zone regime.展开更多
The Naga Hills Ophiolite(NHO)belt in the Indo-Myanmar range(IMR)represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Bu...The Naga Hills Ophiolite(NHO)belt in the Indo-Myanmar range(IMR)represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here,we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO,northeastern India to address(i)the mantle processes and tectonic regimes involved in their genesis and(ii)their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki,Ziphu,Molen,Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine,orthopyroxene,clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting,while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically,studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ)fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf,Zr/Sm,Nb/Ta,Zr/Nb,Nb/U,Ba/Nb,Ba/Th,Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i)a depleted fore arc basalt(FAB)type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii)the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation,fore arc extension and arc-continent accretion.展开更多
基金the funding received from the Science and Engineering Research Board (SERB), Govt. of India under GAP 3291funded by Ministry of Earth Science (MoES) with project reference number Mo ES/ P.O. (Seismic) 8 (09)-Geochron/2012。
文摘Hydrous Cr-bearing uvarovite garnets are rare in natural occurrences and belong to the ugrandite series and exist in binary solid solutions with grossular and andradite garnets. Here, we report the occurrence of hydrous uvarovite garnet having Cr_(2)O_(3) upto 19.66 wt% and CaO of 32.12–35.14 wt% in the serpentinized mantle peridotites of Naga Hills Ophiolite(NHO), India. They occur in association with low-Cr diopsides. They are enriched in LILE(Ba, Sr), LREEs, with fractionating LREE-MREE [avg.(La/Sm)_(N) = 2.16] with flat MREE/HREE patterns [avg.(Sm/Yb)_(N) = 0.95]. Raman spectra indicate the presence of hydroxyl(OH^(–)) peaks from 3500 to 3700 cm^(-1). Relative abundances in fluid mobile elements and their close association with clinopyroxenes are suggestive of the formation of uvarovite garnets through low temperature metasomatic alteration of low-Cr diopsides by hydrothermal slab fluids. The high LREE concentration and absence of Eu anomaly in the garnet further attest to alkaline nature of the transporting slab dehydrated fluid rather the involvement of low-p H solution. The chemical characteristics of the hydroxyl bearing uvarovite hosted by the mantle peridotite of NHO deviate from the classical features of uvarovite garnet, and their origin is attributed to the fluid-induced metasomatism of the sub arc mantle wedge in a suprasubduction zone regime.
基金the funds from Department of Science and Technology,Govt.of India under ECR/2018/000309 project。
文摘The Naga Hills Ophiolite(NHO)belt in the Indo-Myanmar range(IMR)represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene.Here,we present a detailed petrological and geochemical account for the mantle and crustal sections of NHO,northeastern India to address(i)the mantle processes and tectonic regimes involved in their genesis and(ii)their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle.The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki,Ziphu,Molen,Washelo and Lacham areas.The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine,orthopyroxene,clinopyroxene and plagioclase.The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber.Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting,while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE.These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions.Tectonically,studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone(SSZ)fore-arc regime coherent with the subduction initiation process.The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf,Zr/Sm,Nb/Ta,Zr/Nb,Nb/U,Ba/Nb,Ba/Th,Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process:(i)a depleted fore arc basalt(FAB)type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle;(ii)the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration.The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation,fore arc extension and arc-continent accretion.
