Fractional Crystallization Of Biotite Research Articles

Petrogenesis and Tectonic Setting of the Highly Fractionated Junye Granitic Intrusion in the Yiliu Tungsten Polymetallic Deposit, Guangdong Province, South China: Constraints from Geochemistry and Sr-Nd-Pb-Hf Isotopes

The Yiliu tungsten polymetallic deposit, located in the south central portion of the Nanling nonferrous metal metallogenic province in South China, is an area with common Yanshanian tectonothermal events. Early Yanshanian magmatism leads to the emplacement of voluminous tungsten-bearing granite intrusions, such as the Baoshan, Benggangling and Junye plutons, which are considered temporally and spatially associated with W-polymetallic mineralization in the Yiliu region. Here, we investigate the basic geological and petrological characteristics of the Junye granites, and present major and trace element geochemical data and bulk-rock Sr-Nd-Pb-Hf isotopic data to gain insight into the petrogenesis and tectonic setting of granitic intrusions in the region. The Junye granites are high-K calc-alkaline and metaluminous to weakly peraluminous [A/CNK = molar ratios of Al2O3/(CaO + Na2O + K2O) = 0.97–1.02] with enrichment in SiO2 (75.68–76.44 wt.%), relatively high total alkalis (K2O + Na2O = 8.06–8.45 wt.%) with K2O/Na2O ratios ranging from 1.12 to 1.42, and moderate Al2O3 (12.62–13.00 wt.%), but low in P2O5 (<0.01 wt.%), MgO (0.02–0.04 wt.%), CaO (0.78–0.95 wt.%) and Fe2O3T (0.93–1.07 wt.%). They show spectacular tetrad effect REE (rare earth element) patterns with low ΣREE content (53.2–145.3 ppm), negative Eu anomalies (δEu = 0.09–0.17) and slight enrichment of LREEs (light rare earth elements) relative to HREEs (heavy rare earth elements). The granites are enriched in Rb (481–860 ppm), Th (16.2–46.1 ppm) and U (25.4–40.8 ppm) but depleted in Ba (1.0–5.8 ppm), Sr (11.1–23.4 ppm), P (9.5–26.7 ppm) and Ti (241–393 ppm). All geochemical features lead us to interpret the Junye granites as highly fractionated I-type granites. These granites underwent intense interaction between highly evolved magma and volatile-rich hydrothermal fluids during the late stage of formation, and accompanied fractional crystallization of biotite, plagioclase and accessory minerals, such as apatite, monazite and allanite. Additionally, the granites show uniform Nd isotopic ratios with calculated εNd (152 Ma) values of −8.28 to −8.91 and Nd model age (TDM2) of 1645 to 1698 Ma, stable age-corrected initial Pb isotopic compositions with (206Pb/204Pb)i of 18.646–19.010, (207Pb/204Pb)i of 15.767–15.786 and (208Pb/204Pb)i of 39.113–39.159, respectively, and homogeneous Hf isotopic values yielding εHf (152 Ma) values from −6.9 to −9.5 with TDM2 ages of 1680 to 2214 Ma, collectively suggesting that the granitic magma was probably derived from the remelting of ancient infracrustal materials in the basement of the Nanling region. Consequently, we consider that the Junye granites are the products of partial melting of Paleoproterozoic infracrustal medium- to high-K metamorphic basaltic rocks in the Cathaysia Block, which was caused by the underplating of coeval mantle basaltic magmas that provided abundant heat energy for melting in a tectonic setting, with lithospheric extension and thinning during the late Jurassic period.

Biotite in highly evolved granites from the Shimensi W–Cu–Mo polymetallic ore deposit, China: Insights into magma source and evolution

Highly evolved granites are an important type of granitic rock given their close relationship with W–Sn–Nb–Ta and rare-metal mineralization and unusual geochemistry. Compared with typical granites, highly evolved granites experience a more complex magmatic evolution, which is difficult to investigate with whole-rock analyses. In this paper, we present a comprehensive major and trace element (including halogen) dataset for biotites from highly evolved granites in the Shimensi ore deposit, located in northwestern Jiangxi province, South China, which is one of the largest W deposits in the world, to constrain its magma source and evolution.Biotites in the Shimensi granites have a magmatic origin. They are characterized by low Mg# [ = 100 × MgO/(MgO + FeOT), in mol] (29–43, mean = 34), high A/CNK [ = Al2O3/(CaO + Na2O + K2O), in mol] values (1.60–2.91, mean = 1.78), similar to biotite compositions in S-type granites worldwide, suggesting the parental magmas were of meta-sedimentary origin. Biotite trace element data reveal a differentiation trend from fine-grained granite to granite porphyry and porphyritic granite, as indicated by gradually decreasing K/Rb and Nb/Ta ratios and Ba and Ti contents, and increasing Rb, Cs, W, Nb, and Ta contents. Tungsten contents in biotite increase by two orders of magnitude from ∼0.1 ppm in fine-grained granite to ∼30 ppm in porphyritic granite due to progressive fractional crystallization. The log (fH2O/fHF) and log (fHCl/fHF) values inferred from the biotites increase with progressive fractional crystallization, indicating no obvious fluid exsolution before or during biotite crystallization. High degrees of fractional crystallization and late fluid exsolution in the Shimensi granites were favorable for W mineralization. Nb/Ta ratios of the biotites decreased from 24.3 to 4.30 with progressive fractional crystallization. This result, along with the Rayleigh fractional crystallization modeling results for whole-rock Nb and Ta contents, suggest that fractional crystallization of biotite is the major control on decreasing Nb/Ta ratios in the Shimensi granites.

Geochemical and isotope evidence for mantle-derived source rock of high-K calc-alkaline I-type granites, Pernambuco–Alagoas Domain, northeastern Brazil

We report whole-rock major, trace, mineral, and isotope chemistries of the Jacare dos Homens, and Santo Antonio granitic rocks from the Aguas Belas–Caninde batholith, Pernambuco–Alagoas Domain of the Borborema Province, northeastern Brazil. These rocks exhibit low-angle foliation, suggesting emplacement under a regional strain field, associated with transpressive deformation tied to the onset of the Brasiliano orogeny. Both rocks are dominantly made up of K-feldspar, quartz, and plagioclase with biotite as the main mafic phase and minor hornblende. The Jacare dos Homens orthogneiss displays magnesian, slightly peraluminous, and alkali-calcic character, while the Santo Antonio granite is magnesian, metaluminous to slightly moderately peraluminous and alkali-calcic to alkalic. These granitic rocks are enriched in alkalis (up to 10.28 wt%), mg#, Rb, Ba, Sr, Th, and LREE, and depleted in HREE and HFSE, with distinct Nb, Ta, and Ti depletion. Decreasing MgO, CaO, TiO2, Na2O, Fe2O3T, P2O5, Al2O3, Sr, Zr, and Hf with increasing SiO2 are consistent with fractional crystallization of biotite, amphibole, plagioclase, apatite, zircon, titanite, and Fe–Ti oxides. U–Pb zircon dating of the Jacare dos Homens and Santo Antonio rocks yielded concordia ages of 642.4 ± 3 Ma and 636 ± 4 Ma, respectively. The isotope data for the Jacare dos Homens orthogneiss yielded an Nd-model age of 1.21 Ga with slightly negative eNd(642 Ma) of − 1.58, initial 87Sr/86Sr ratios of 0.7068, and δ18O (zircon) value of + 7.0‰ VSMOW. The isotope data for Santo Antonio pluton yielded Nd-model ages between 0.96 and 1.07 Ga, with eNd(636 Ma) from + 1.17 to − 0.67, initial 87Sr/86Sr ratios from 0.7048 to 0.7056, and δ18O (zircon) value of + 5.0 to 5.9‰ VSMOW. Altogether, the petrological, geochemical, and isotopic data are typical of I-type granites. We propose that the studied granitic rocks were derived from mantle-derived medium- to high-K basaltic lower crust of Stenian/Tonian (Jacare dos Homens) or Tonian (Santo Antonio) ages and that partial melting was triggered by the uplift of asthenosphere and underplating of lithospheric mantle. The migration and buoyancy-driven ascent of the magma to their present level were probably favored by the Jacare dos Homens shear zone.

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province, South China: Constraints from Zircon U‐Pb Geochronology, Geochemistry and Nd Isotopes

Abstract:Mesozoic granitic intrusions are widely distributed in the Nanling region, South China, Yanshanian granites are closely connected with the formation of tungsten deposits. The Xihuashan granite is a typical representative of tungsten‐bearing granite. The Xihuashan granite consists mainly of medium‐grained porphyritic biotite granite, medium‐grained biotite granite and fine‐grained two‐mica granite, which correspond to LA‐ICP‐MS zircon U‐Pb ages of 155.5±0.4 Ma, 153.0±0.6 Ma and 152.8±0.9 Ma, respectively. Rocks from the Xihuashan mining area displays high SiO2 (73.85% to 76.49%) and Na2O+K2O contents (8.09% to 9.43%), belonging to high‐K calc‐alkaline series. They are metaluminous to weakly peraluminous with A/CNK values ranging from 0.96 to 1.06. All granites in this study area are rich in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti, Nb and Eu, especially depleted in medium‐grained biotite granite and fine‐grained two‐mica granite. The medium‐grained porphyritic biotite granites usually have high LREE concentrations, whereas medium‐grained biotite granite and fine‐grained two‐mica granite displays high HREE contents. Our geochemical data reveal that the studied rocks are highly fractionated I‐type granite. The magma underwent strong magma differentiation with decreasing temperature and increasing oxygen fugacity, which may explain the formation of three types of distinct granites. Variations of Rb, Sr and Ba concentrations in different type granites were controlled by fractional crystallization of biotite and feldspar. Fractional crystallization of monazite, allanite and apatite resulted in LREE changes in granite, and formation of garnet mainly caused HREE changes. Granites from the Xihuashan mining area have relatively high εNd(t) values (–9.77 to –11.46), indicating that they were probably generated by partial melting of underlying Proterozoic metasedimentary rocks with minor addition of juvenile crust or mantle‐derived magmas.

Geochemistry of the rare metal-bearing pegmatite No. 3 vein and related granites in the Keketuohai region, Altay Mountains, northwest China

The chemical composition of a barren biotite granite, a two-mica granite, and a rare metal-bearing pegmatite (the No. 3 vein in Keketuohai, Altay Mountains, northwest China), along with mineral separates of apatite and mica, were analyzed and compared in the present work. Bulk chemistries of biotite granite, two-mica granite, and pegmatite rim are consistent with fractional crystallization trend from granites toward pegmatite. Changes in pegmatite composition can be explained mainly by fractional crystallization of biotite. Rb–Sr isochrons yield 248.8±7.5 Ma for the biotite granite, 247.8±6.3 Ma for the two-mica granite, and 218.4±5.8 Ma for the pegmatite rim. Similar geochemical characteristics, including initial ε Nd(T) values (ranging from −0.76 to −3.04) for apatites, similar initial ε Nd(T) values for whole-rock samples of studied granites and pegmatite (ranged from −1.40 to −3.21) and for biotites from granites (ranged from −2.75 to −3.15) suggest that these rocks were derived from a common magma source. Old continental crust may have played a significant role in magma generation. Based on these data, we interpret the evolution of the Keketuohai granite–pegmatite by magma differentiation from a common source. Biotite granite was the first to crystallize, followed by two-mica granites. The residual melt was probably stored in a granitic batholith. The pegmatite veins were injected ∼30 M.y. later during another episodic tectono-magmatic event in the Altay Mountains.

Petrology of the South Mountain Batholith, Nova Scotia

Approximately one-third of western Nova Scotia is underlain by granitic rocks of the South Mountain batholith. The pi u ton is an epizonal complex of granodiorite, adamellite and alaskite which was emplaced after the folding and metamorphic events of the Acadian orogeny. Chemical and petrographic data suggest that the rocks comprise a single comagmatic suite related by the fractional crystallization of biotite and plagioclase. The magma for the batholith may have been produced by partial fusion of the lower crust, or through a hybridization of mantle and crustal melts.