Geochemistry and SHRIMP Zircon U‐Pb Age of Post‐Collisional Granites in the Southwest Tianshan Orogenic Belt of China: Examples from the Heiyingshan and Laohutai Plutons
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Abstract: The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A‐type granites geochemically. Analysis shows that the both are ferron calc‐alkalic peraluminous or ferron alkali‐calcic peraluminous with a relatively high concentration of SiO 2 (>70%), high alkali contents (Na 2 O + K 2 O = 7.14%‐8.56%; K 2 O>N 2 O; A/CNK = 0.99‐1.20), and pronounced negative anomales in Eu, Ba, Sr, P and Ti. A SHRIMP zircon U‐Pb age of 285±4 Ma was obtained for the Heiyingshan hornblende biotite granite intrusion. The geochemical and age dating data reported in this paper indicate that these granites were formed during the post‐collisional crustal extension of the Southwest Tianshan orogenic belt, in agreement with the published data for the granites in the South Tianshan.Keywords:
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K-Ar mineral ages have been determined for late Cretaceous to early Tertiary granitic plutons in Ulsan-Kyeongju area, the mid-eastern Kyeongsang Basin. The granitic plutons in the area can be grouped into two plutonic units; the Ulsan and Kyeongju plutons. The Ulsan plutons consist of hornblende biotite granodiorite, Ulsan granitic complex and porphyritic biotite granite, and the Kyeongju plutons are composed of hornblende biotite granodiorite, biotite granite and alkali granite. K-Ar biotite ages (67∼51 Ma) of the Ulsan plutons are significantly older than those (50∼47 Ma) of the Kyeongju plutons. Combined with the previous results about the cooling history of adjecent plutons and petrological information of the studied plutons, Ulsan hornblende biotite granodiorite and Ulsan granitic complex, which have K-Ar biotite ages older than 62 Ma, are thought to have been emplaced during late Cretaceous. On the other hand, Ulsan porphyritic biotite granite, Kyeongju biotite granite and Kyeongju alkali granite, which show K-Ar biotite ages younger than 54 Ma, seem to have been formed during early Tertiary. However, it is impossible to infer the emplacement age of Kyeongju hornblende biotite granodiorite, because its K-Ar biotite ages are considered to have been reset by later thermal effect. Considering the existence of early Tertiary plutons in the study area, particularly Kyeongju area, is likely to be the youngest granitic province or segment in the Kyeongsang Basin.
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The present study aims at understanding how humic acids (HAs) (extracted from Entisol, Alfisol and Mollisol) affect and transform primary silicate minerals, viz., hornblende (inosilicate) and biotite (phyllosilicate), and aid in nutrient release therefrom. The focus is on the kinetics of dissolution and the alterations brought about in the products formed after treatment with HA. The kinetic curves exhibit alternate crests and troughs pointing to alternate solubilisation and precipitation. Structural difference between the two primary silicates lead to differential release of cations. The contrast between the minerals is more pronounced when the altered products are compared. While hornblende shows drastic changes including reduction in Fe2+/3+ and Ca2+ and enrichment in Si4+, Al3+ and Mg2+, biotite is almost minimally altered. XRD(X ray diffraction) studies indicate the disappearance/attenuation of many prominent bands of hornblende in the weathered residues with several new crystalline phases. Biotite, however, is only slightly altered with the formation of a few new phases. IR studies suggest deposition of HA on the surfaces of both biotite and hornblende but such deposition is far more prominent with hornblende.
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Biotite and hornblende from the western portion of the Cortlandt complex record undisturbed $$^{40}Ar/^{39}Ar$$ incremental-release spectra with average total-gas dates of 420 m.y. (hornblende) and 390 m.y. (biotite). Biotite from the younger phase of the Rosetown pluton also records undisturbed release spectra with an average total-gas date of 420 m.y. Coexisting hornblendes display more complex spectra (a result of excess argon contamination); however, they have similar high-temperature "plateaus" at 450 m.y. Both biotite and hornblende from the older portion of the Rosetown have disturbed release spectra, possibly a result of excess argon contamination (hornblende) and partial postcrystallization gas loss (biotite). The disturbed spectra preclude a definitive estimate of absolute age but the data do suggest a minimum date of ~485-500 m.y. Because field criteria (Ratcliffe 1968, 1971) clearly indicate that both the Cortlandt and the younger Rosetown postdate a regionally pervasive progressive metamorphism, it is suggested that their differences in geochronology reflect regional variations in times of postmetamorphic cooling and that the 450-m.y. date defined by the Rosetown hornblendes closely approximates the time of intrusion of both plutons. This conclusion requires that the regional metamorphic event was Taconic in age. The undisturbed incremental release spectra of the Cortlandt samples indicate no substantial argon loss which could be attributed to Acadian reheating. This substantiates the conclusion that regional metamorphic zonation is Taconic rather than Acadian, and suggests that this area was little affected by Devonian or later reheating. The difference in probable age of the older part of the Rosetown (485-500 m.y.) and the cross-cutting younger phase (450 m.y.) bracket a period of reactivation of the Ramapo fault system and suggests that fault activity was concurrent with Middle Ordovician Taconic metamorphism.
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A group of slightly peraluminous Variscan plutons in Northern Portugal were selected from the study of zircon composition. The selected plutons are: the Vila Pouca de Aguiar and the Lavadores-Madalena plutons with I-type affinities and the Vieira do Minho pluton, an l-S transitional type. Zircon occurs as euhedral to subhedral crystals and exhibit finely concentric oscillatory magmatic zoning mainly related to variations of Hf, Y, U and Th concentrations. Most zircon crystals show the dominant “xenotime” substitution. The zircon crystals have Zr/Hf ratio in the range of 21 to 52, with no significant differences between the different granites. These values are in the same range of other peraluminous granites and are in accordance with a crustal signature of zircon. Moreover, the range of Zr/Hf valu es in zircon crystals overlaps with that of crustal sources and consequently to the potential protoliths proposed in the genesis of the Vieira do Minho and the Vila Pouca de Aguiar plutons, namely meta-igneous crustal sources at different levels. Although zircon from the Lavadores-Madalena pluton has a compositional range similar to the other plutons, an origin by hibridisation has been proposed. However, similar zircon chemistry between this pluton and Vila Pouca de Aguiar and Vieira do Minho plutons could also suggest a similar crustal source.
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1974)
The deuterium content (δDSMOW) of the water extracted from coexisting biotite and hornblende in granitic rocks was measured. The relationship between δD of biotite and hornblende suggested an isotopic equilibrium in the rocks of four granitic bodies in the Kitakami mountains (Northeast Japan), and the maximum temperature of isotopic equilibration of biotite and hornblende was estimated to be 700-1, 000°C, provided that the minimum temperature was 450-550°C. The δD value obtained for the water with which these minerals equilibrated was -29_??_-37‰ and is definitely enriched in deuterium relative to the present local surface water. On the other hand, the relationship between δD of biotite and hornblende in the Ryoke metamorphic belt contiguous to the Median Dislocation Line (Southwest Japan) indicated an isotopic disequilibrium. In this case a complicated history of formation of these granitic rocks in terms of the interaction of water and hydrous silicates was inferred.
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40Ar/39Ar analyses on hornblende and biotite samples separated from the Tanzawa tonalite demonstrate that most samples have highly disturbed K-Ar systematics. Expect for Mt. Ishiwari samples, unrealistically old ages are commonly observed in lower temperature fractions. Especially, Omatazawa samples showed severely disturbed age spectra, which strongly suggests that one should not use the previously reported 10 Ma hornblende K-Ar ages in the southwestern margin of the Tanzawa pluton in a discussion of the cooling history of the pluton. In the eastern part, although showing a clear evidence of excess argon in the lower temperature fractions, the biotite and hornblende in the Yushin sample yield poorly defined high temperature plateaus about 5 Ma. On the other hand, the biotite and hornblende separated from the Mt. Ishiwari sample reveal apparently good plateaus in age spectra. However, the biotite has a plateau age older than that for the cogenetic hornblende, which is consistent with our previous K-Ar results on these samples, but is inconsistent with the widely accepted closure temperature hypothesis. Comparing with our previous K-Ar results, the 6.26 +/- 0.17 plateau age for the hornblende could be interpreted as the time when the sample cooled to the closure temperature of hornblende (about 500°C). The older age of the biotite should be due to excess argon.
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