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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
正式出版
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    Episodic bimodal magmatism in Central Inner Mongolia,China: Insights from geochronological,geochemical,and Sr-Nd isotopic evidence

    Junbin Zhu
    15页
    查看更多>>摘要：This study combines zircon U-Pb ages,whole-rock major and trace elements,and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt.The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian.The late Carboniferous(ca.323 Ma)bimodal associations consist mainly of gabbroic and granitic intrusive rocks,whereas the early Permian(ca.273 Ma)bimodal suites are dominated by dolerite and granite porphyry dikes.The gabbroic intrusive rocks and dolerite dikes are calc-alkaline,slightly enriched in light rare earth elements and depleted in Nb,Ta,Ti,and have low initial ~(87)Sr/~(86)Sr ratios(0.70317-0.70510)and high ε_(Nd)(t)values(+3.1 to +7.4).They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components.The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks,suggesting they were derived from a melt-induced anatexis of basaltic lower crust.The early Permian granite porphyry dikes show characteristics of LREE enrichment,negative Nb-Ta anomalies and positive Zr-Hf anomalies.They have higher La/Yb and Th/Zr ratios and ε_(Nd)(t)values,and lower initial ~(87)Sr/~(86)Sr ratios than the coeval dolerite dikes.These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components.An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a post-collisional to within-plate rift setting after the closure of the Paleo-Asian Ocean.
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    • NSTL

    Petrogenesis and geodynamic mechanisms of the Late Cretaceous magmatic'flare-up'in the southern Lhasa Terrane.,Tibet

    Yamei TianFeng HuangJifeng Xu
    16页
    查看更多>>摘要：Exploring magmatic'flare-up'events can be used to unravel dynamic Earth processes including plate subduction and crustal growth.However,the cause and mechanism of magmatic'flare-up'remain unresolved.The abundant magmatic records that are preserved in the Lhasa Terrane on the southern Tibetan Plateau make it an important region to study the episodic nature of the secular magmatism.This paper presents geochronological,mineral-ogical,and geochemical data that describes the Dazhuka diorites in the central-western and Sangri granites in the eastern parts of the southern Lhasa Terrane.Zircon LA-ICP-MS U-Pb data indicate that these rocks were generated around ~95-90 Ma.Both are enriched in large ion lithophile elements and depleted in high field strength elements,indicating geochemical affinity with subduction-related magmatic rocks.The Dazhuka diorites have low SiO2 contents(54.49-56.72 wt%),with moderate Mg#(41.7-42.7)and rare earth element fractionation((La/Yb)N = 6.9-8.6),and homogeneous whole-rock ~(87)Sr/~(86)Sri(0.704204-0.704480),ε_(Nd)(t)(+3.5-h4.1),and zircon ε_(Hf)(t)(+1_(0.2)-+13.3).These features suggest that the Dazhuka diorites originated in the spinel-garnet transition zone within the lithospheric mantle of a mantle wedge environment.The Sangri granites have high SiO2(64.36-69.54 wt%),Al2O3(14.27-15.61 wt%),and(La/Yb)N(4.6-49.6)and Sr/Y(34.7-119.3)ratios,indicating geochemical affinity with adakites.Low MgO(1.3-2.22 wt%)and depleted Sr-Nd-Hf isotopes(whole-rock ~(87)Sr/~(86)Sri = 0.704099-0.705453,ε_(Nd)(t)= +2.9-+3.5,zircon ε_(Hf)(t)= +7.1-+9.3)suggest that the granites were derived from oceanic crust.The types of magmatic rocks that are distributed over the central-western and the eastern parts of the southern Lhasa Terrane differ,with higher magmatic temperatures in the east as compared to the central-west.These results indicate that different areas in the southern Lhasa Terrane may have experienced different geodynamic processes,with oceanic ridge subduction in the eastern part of the southern Lhasa Terrane and normal oceanic plate subduction in the central-western part.These different types of subduction processes are likely to have caused the magmatic'flare-up'and variation in the magmatism of southern Tibet.
      原文链接:
    • NSTL

    Anatexis and resultant magmatism of the Ama Drime Massif: Implications for Himalayan mid-Miocene tectonic regime transition

    Daxiang GuJinjiang ZhangChao Lin
    17页
    查看更多>>摘要：In the middle Miocene,the Himalayan Orogen underwent a remarkable tectonic regime transition from a north-south extension to an east-west extension,but the underlying mechanisms of the Himalayan mid-Miocene tectonic regime transition are still debated.In this study,we conducted an integrated petrological,geochronolog-ical,and geochemical study on the high-grade metamorphic rocks and leucogranites from the Ama Drime Massif(ADM)to detect the anatexis and magmatism associated with the east-west extension.According to geochro-nological results,the structurally higher ADM paragneisses and the structurally lower ADM orthogneisses underwent partial melting in 22-12.5 Ma and 14-13 Ma,respectively,accompanied by the formation of 12 Ma tourmaline-muscovite leucogranites and 12-11 Ma two-mica leucogranites.By comparison,the tourmaline-muscovite leucogranites and the ADM paragneisses have compatative protolith ages and Sr-Nd isotopic compositions.Likewise,the two-mica leucogranites and the ADM orthogneisses are comparable in protolith ages and Sr-Nd isotopic compositions.For whole-rock geochemistry,the two-mica leucogranites are characterized by higher TiO2,TFeO,MgO,CaO,Sr,Ba,LREE,Th,Zr/Hf,but lower Rb,Rb/Sr,and weak Eu anomalies compared to the tourmaline-muscovite leucogranites.Based on the spatial,temporal,and protolith relationships between the gneisses and leucogranites in the ADM,the tourmaline-muscovite leucogranites originated from muscovite dehydration melting of the paragneisses and then experienced strong fractional crystallization,while the two-mica leucogranites resulted from biotite dehydration melting of the orthogneisses.Anatexis and resultant magmatism in the ADM reveal that the mid-Miocene tectonic regime transition changed not only the style of the Himalayan extensional structures but also the crustal thermal structure of the Himalayan Orogen.In this contribution,a potential lateral delamination model is proposed to accommodate the heating and coeval rapid east-west extension in the ADM region and to explain the Himalayan mid-Miocene tectonic regime transition-
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    Unradiogenic lead isotopic signatures of the source mantle beneath the southernmost segment of the Central Indian Ridge

    Hiroshi SatoShiki MachidaKana Ashida
    13页
    查看更多>>摘要：Numerous studies have shown that the mid-ocean-ridge basalts(MORBs)distributed in the southern Central Indian Ridge(CIR)exhibit the DUPAL anomaly; yet,the corresponding temporal variations of this signal remain underexplored in this region.Here,we report new Sr-Nd-Pb isotope data for the MORB which forms the southernmost segment of the CIR(CIR-1)and interpret these together with previously reported elemental data.Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust(LCC).This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly.In contrast,the MORB from the of f-axis area shows a depleted trace element composition,particularly for highly incompatible elements.This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component.Furthermore,high ~(87)Sr/~(86)Sr,A8/4,and A7/4 values indicate that the source mantle of the MORB in the of f-axis area of the CIR-1 segment has undergone at least two melting events,including the formation of the current MORB.The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios(Th/Pb and U/Pb)in the residual mantle,yielding an unradiogenic lead isotope composition.Overall,our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment,contributing to the formation of MORBs with an unradiogenic lead isotopic signature.
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    • NSTL

    Porphyry Cu fertility of eastern Paleo-Tethyan arc magmas: Evidence from zircon and apatite compositions

    Jing-Jing ZhuRuizhong HuXian-Wu Bi
    14页
    查看更多>>摘要：The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood.It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity.We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region,SW China,represented by the Maoding(268.3 ± 2.3 Ma),Suwalong(275.6 ± 1.4 Ma to 264.7 ±1.4 Ma)and Baimaxueshan(255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma)granitoids.They have geochemical features consistent with typical continental arc rocks,formed from partial melting of the mantle wedge,with increasing contamination of crust-derived melts over time.The presence of amphibole,high Sr/Y ratios(35 ± 20,n = 17),and lack of negative Eu anomalies(Eun/Eun* = 1.04 ± 0.3,n = 17)are consistent with derivation from hydrous magmas.Zircons from the intrusive rocks have relatively low Eun/Eun*(mostly<(0.4))ratios and calculated AFMQ values(-2.1 ± 1.2,n = 86).Combined with apatite microphenocrysts with extremely low sulfur contents(<0.12 wt% SO3),it suggests that the intrusions originated from water-rich but reduced arc magmas,that were less fertile for porphyry Cu formation.Our study highlights that magmatic fO2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility.
      原文链接:
    • NSTL