Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26° and 29° south
Robert H. McNuttJ. H. CrocketAlan H. ClarkJuan C. CaellesEdward FarrarSimon J. HaynesMarcos Zentilli
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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 oldest igneous rocks in the Comstock District are amphibolites probably derived from basalts and of Triassic age. These are intruded by pre‐Tertiary quartz monzonlte and by granodiorite of Sierran facies, the latter not being exposed on the surface but found on mine‐dumps. Igneous activity recorded mainly in volcanic rocks was almost continuous throughout the Tertiary. Its products, in order of age, were as follows: Eocene—Rhyolitic flows aggregating a few hundred feet in thickness: One intrusive plug of rhyolite occurs. Eocene or Miocene—Hornblende‐auglte andesite intrusive into the rhyolite, type locality American Ravine.
Basaltic andesite
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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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The relations of Yellowstone Tertiary volcanics to Tertiary faults in the Teton Range indicate that major movements probably occurred in early Pliocene time, following deposition of Miocene basic volcanics and associated sediments and prior to outpourings of plateau rhyolite, and that minor movements continued into Recent time. Erosion surfaces representing the late-mature stage Black Rock cycle are covered by the rhyolite. This relationship and the youthful geomorphic aspect of the rhyolite plateau indicate that the rhyolite and younger valley basalt are probably of early, or early middle, Pleistocene age. More precise dating of the rhyolite depends upon the age assigned to the Buffalo drift, which rests on these volcanics.
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Baijiangou is in the Kelameili mountain of eastern Junggar Basin.The Carboniferous volcanic rocks in the Baijiangou is various,such as basalt,andesibasalt,andesite,rhyolite and volcanic brecci.The Carboniferous volcanic rocks of west Baijiangou stand for the rock sequence of lower Bashan group(C2b),while the the Carboniferous volcanic rocks of east Baijiangou stand for the rock sequence of upper Bashan group(C2b).The whole sequence of Baijiangou outcroup includes basalt andesite rhyolite.This sequence of vocanic rocks is in accord with the magma evolution law and the well logging,and could be as a guide for the exploration of Carboniferous volcanic rocks.
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The relations of Yellowstone Tertiary volcanics to Tertiary faults in the Teton Range indicate that major movements probably occurred in early Pliocene time, following deposition of Miocene basic volcanics and associated sediments and prior to outpourings of plateau rhyolite, and that minor movements continued into Recent time. Erosion surfaces representing the late-mature stage Black Rock cycle are covered by the rhyolite. This relationship and the youthful geomorphic aspect of the rhyolite plateau indicate that the rhyolite and younger valley basalt are probably of early, or early middle, Pleistocene age. More precise dating of the rhyolite depends upon the age assigned to the Buffalo drift, which rests on these volcanics.
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