Oligocene magmatism in the eastern margin of the east Himalayan syntaxis and its implication for the India–Asia post-collisional process
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The Tuncang–Chuzhou–Machang area (eastern Anhui province) is geologically located in the intersection between the Yangtze block and the Qinling–Dabie orogenic belt. Many Mesozoic plutons outcrop in this district that are Cu–Au prospective but inadequately studied. We report new LA-ICP-MS zircon U–Pb ages, petrologic, and whole rock geochemical data for three representative plutons at Machang, Huangdaoshan, and Tuncang. New dating results suggest that all the Machang (129.3 ± 1.6 Ma), Huangdaoshan (129 ± 1.7 Ma), and Tuncang (130.8 ± 1.9 Ma) plutons were emplaced in the Early Cretaceous, slightly older than other plutons in neighbourhood of the Zhangbaling uplift. The three plutons contain typical low-Mg adakitic affinities, in which the rocks contain SiO2 >56%, Al2O3 ≥15%, Mg# <53, elevated Sr, Ba, Cr, Ni, Sr/Y, and La/Yb, low Y and Yb and no discernible Eu anomaly. Their petrogenesis may have been related to the delamination and partial melting of the lower crust, which is different from the Chuzhou pluton, which was interpreted to have formed by partial melting of the subducted slabs. We suggest that this petrogenetic difference may explain why the pluton at Chuzhou is Cu–Au fertile, whereas those at Machang, Huangdaoshan, and Tuncang are largely barren. It is proposed that adakitic plutons formed by partial melting of the subducted slabs have high metallogenetic potentiality in the area.
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The common association of mid-crustal migmatites with an upper-level granite pluton could indicate that the migmatites are a feeder zone for the pluton. If magma from a deeper level pervasively intrudes a high temperature metamorphic complex, most of the intruded magma would not freeze because of the prevailing temperature. The interaction between the magma and country rocks, which could include partial melting and crystallisation of the magma passing through, would modify magma to a more granitic composition, as found in the higher-level pluton.
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Granitoid plutons are a major component of pre-Carboniferous rocks in Cape Breton Island and knowledge of the time and tectonic setting of their emplacement is crucial for understanding the geological history of the island, guiding exploration for granite-related economic mineralization, and making along-orogen correlations. The distribution of these plutons and their petrological characteristics have been used in the past for recognizing both Laurentian and peri-Gondwanan components in Cape Breton Island, and for subdividing the peri-Gondwanan components into Ganderian and Avalonian terranes. However, ages of many plutons were assumed on the basis of field relations and petrological features compared to those of the relatively few reliably dated plutons. Seventeen new U–Pb (zircon) ages from igneous units reported here provide enhanced understanding of the distribution of pluton ages. Arc-related plutons in the Aspy terrane with ages of ca. 490 to 475 Ma likely record the Penobscottian tectonomagmatic event recognized in the Exploits subzone of central Newfoundland and New Brunswick but not previously recognized in Cape Breton Island. Arc-related Devonian plutonic activity in the same terrane is more widespread, continuous, and protracted (445 Ma to 395 Ma) than previously known. Late Devonian magmatism in the Ganderian Aspy terrane is similar in age to that in the Avalonian Mira terrane (380 to 360 Ma) but the tectonic settings are different. In contrast, magmatic activity in the Bras d’Or terrane is almost exclusively arc-related in the Late Ediacaran (580 to 540 Ma) and rift-related in the Late Cambrian (520 to 490 Ma). The new data support the terrane distinctions previously documented.
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Neoproterozoic magmatism in the Hannan region at the northwestern margin of the Yangtze Block is characterized by numerous felsic plutons associated with minor mafic-ultramafic intrusions. The felsic plutons are either adakitic or normal-arc granitic in composition. The adakitic plutons are ∼735 Ma in age and are interpreted as having formed by partial melting of a thickened lower mafic crust. Among the normal-arc-related felsic plutons, the Tianpinghe pluton is the largest and has a SHRIMP zircon U-Pb age of 762 ± 4 Ma, older than the adakitic plutons in the region. Rocks from the Tianpinghe pluton have relatively high SiO (67.1–70.1 wt%) and KO + NaO (7.8–8.6 wt%) and relatively low MgO (0.7–1.3 wt%) and AlO contents (14.5–15.6 wt%), with AlO/(CaO + KO + NaO) (A/CNK) values ranging from 0.95 to 1.08. They have arc-affinity trace-element compositions that are characterized by enrichment of large-ion lithophile elements and depletion of high-field-strength elements (Nb, Ta), with strong positive Pb and negative Ti anomalies. They have a narrow range of εNd values (+0.15 to -1.76) and relatively high zircon εHf values (+0.6 to +8.3). These geochemical features are typical of I-type granites. The rocks from the Tianpinghe pluton have relatively young single-stage and two-stage Hf model ages (1.01–1.31 and 1.31–2.01 Ga, respectively), suggesting that the pluton was generated by partial melting of newly formed basaltic rocks. On the basis of its arc-related geochemical affinity and its emplacement before voluminous adakitic magmatism but after mafic-ultramafic intrusions, the Tianpinghe pluton is considered to be Neoproterozoic arc granite formed during a period of crustal growth and reworking. Generation of the later adakitic plutons suggests that the crustal thickness increased to more than 50 km by mafic magma underplating.
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Based on the great thickness crustal struture and complex geophysical character,this paper points out the principle for dividing terranes in Qinghai─Xizang Plateau according to earthquake activities and wave field sign,lithospheric structure and velocity character,palemagnetic mark,potential field sign ,temperature sign, geological and tectonics character. From north to south of the Qinghai─Xizang Plateau and its neighbouring regions can be devided in seven terranes, ie, Qaidam terrane, Kunlun terrane, Hon Xil-Bayan Har terrane, Qangtang terrane, Lhasa-Gangdise terrane, Himalayan terrane and Ganges Plain terrane. The distribution and character of these terranes have important rools for the studies of the formation and evolution.of the plateau and plate movement and dynamic mechanism.
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Rubidium and strontium concentrations and (87)Sr/(86)Sr values are documented herein for samples of plutons and associated rocks from 238 locations in the southern Sierra Nevada and vicinity. The goals of the investigation were to determine ages of rock units, to aid in the separation of plutons in poorly exposed areas, to determine the pattern of variation of initial (87)Sr/(86)Sr (hereafter called Sr(i)) for plutons, and to constrain more rigorously the boundaries of the continental Sierran and Salinian-western Mojave terranes defined on the basis of the Sr(i) of their plutons by Kistler (1978) and by Kistler and Peterman (1978). These new data expand the boundaries of the Salinian-western Mojave terrane and make it part of the Panthalassan lithosphere that lies west of the tectonic boundary, whereas the Sierran terrane is made part of the North American lithosphere that lies east of the tectonic boundary.
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Field and geochronologic evidence indicate that large and broadly homogeneous plutons can accumulate incrementally over millions of years.This contradicts the common assumption that plutons form from large, mobile bodies of magma.Incremental assembly is consistent with seismic results from active volcanic areas which rarely locate masses that contain more than 10% melt.At such a low melt fraction, a material is incapable of bulk flow as a liquid and perhaps should not even be termed magma.Volumes with higher melt fractions may be present in these areas if they are small, and this is consistent with geologic evidence for plutons growing in small increments.The large melt volumes required for eruption of large ignimbrites are rare and ephemeral, and links between these and emplacement of most plutons are open to doubt.We suggest that plutons may commonly form incrementally without ever existing as a large magma body.If so, then many widely accepted magma ascent and emplacement processes (e.g., diapirism and stoping) may be uncommon in nature, and many aspects of the petrochemical evolution of magmatic systems (e.g., in situ crystal fractionation and magma mixing) need to be reconsidered.
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Based on abundant geological data,this paper divides geologically Fujian province into four distinct terranes,namely,Northwest Fujian terrane,Southwest Fujian terrane,East Fujian terrane and Southeast Fujian terrane,and correlates their geological features in detail respectively.Through the study on collision-amalgamation history of each terrrane,it can be concluded that they mainly underwent three stages:(1)in Late Proterozoic,the Southwest Fujian terrane and Northwest Fujian terrane collided and amalgamated along the Nanping-Ninghua fracture zone,being accompanied by submarine and continental volcanic eruptions;(2)in Triassic,the East Fujian terrane collided and amalgamated with the Southwest Fujian terrane and Northwest Fujian terrane along the Zhenghe-Dapu fracture belt,and the Southeast Fujian terrane amalgamated simultaneously with the East Fujian terrane;(3)in Cretaceous,the Southwest Fujian terrane underwent a left-lateral strike slip along the Pingtan-Dongshan fracture zone.
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In southern Alaska, multiple terranes—slabs that have broken off from larger tectonic plates and shuffed around—create a complex patchwork that makes it challenging for scientists to untangle the tectonic history and structure of the region. One of these, the Yakutat terrane, which lies just offshore southern Alaska in the Gulf of Alaska, is converging with the North American plate and driving the growth of the Chugach–St. Elias mountains. The structure of this terrane has not been well studied until now. Worthington et al. conducted seismic studies to create a two‐ dimensional seismic velocity model of the Yakutat terrane. The model allows them to constrain the crustal thickness and composition of the terrane.
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