Petrogenesis Of A-type Granites Research Articles

Two Distinct Fractional Crystallization Mechanisms of A-Type Granites in the Nanling Range, South China: A Case Study of the Jiuyishan Complex Massif and Xianghualing Intrusive Stocks

The petrogenesis of A-type granites with different occurrences in the Nanling Range remains unclear. In this study, a case study of the Jiuyishan complex massif and Xianghualing intrusive stocks was conducted to determine this problem. The Jiuyishan complex massif is composed of four units (Jinjiling, Pangxiemu, Shaziling and Xishan). These four units have similar zircon U-Pb ages of approximately 153 Ma, with high Zr + Nb + Ce + Y contents (>350 ppm), high 10,000 Ga/Al ratios (>2.6), and a high crystallization temperature, indicating A-type affinities. They show a gradual change in lithology and geochemistry, implying a fractional crystallization process. These units also have similar εNd(t) values (−8.2 to −5.8) and zircon εHf(t) values (−7.5 to −2.2) except for the Shaziling MMEs (mafic microgranular enclaves) (−14.2 to 4.8), demonstrating their lower crustal source. However, the Shaziling unit may have contributed mantle-derived magma based on the geochemical data of its hosted MMEs. In comparison, the two Xianghualing intrusive stocks have similar geochemical features but exhibit highly evolved features (high Rb, U, Y, Ta and Nb contents and low Eu, Ba, Sr, P, Ti, Ca, Mg and Fe contents, with V-shaped REE distribution patterns). They have different zircon U-Pb ages of approximately 160 Ma and 155 Ma. The two stocks also have similar whole-rock εNd(t) values (−6.5 to −5.7) and zircon εHf(t) values (−7.6 to −2.7) and equally illustrate a lower crustal source region. Combining with their vertical zonation, they may have experienced remarkable fractional crystallization with possible assimilation processes. We propose that the Jiuyishan complex and Xianghualing stocks have two distinct fractional crystallization mechanisms during their formation. The Jiuyishan complex was formed by in situ crystal mush fractionation, while the Xianghualing stocks were formed by flowage differentiation during magma ascent or gravitational settling during magma solidification after emplacement. However, more than one mechanism affected the fractional crystallization processes of these granitic rocks.

Petrogenesis of the early Permian A-type granites in the Halajun region, southwest Tianshan, western Xinjiang, NW China: implications for geodynamics of Tarim large igneous province

ABSTRACT Formation of a large igneous province, an explosive magmatic event in earth, is intimately related to geodynamic processes. The Tarim large igneous province, a famous representative in China, contains A-type granites. However, the petrogenesis of the A-type granites is debated. Some researchers think that the A-type granites are developed through extensive fractionation of magma derived from partial melting of newly underplated or gabbroic rocks. Others insist that the A-type granites are developed through intensive fractional crystallization of plume-derived mafic magma. Here, we present zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotopic data for the Kezilekezitage, Keziertuo, Tamu, and Halajun II intrusions in northwest Tarim to elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating yielded crystallization ages of the Kezilekezitage, Keziertuo, and Halajun II intrusions of 275.1 ± 3.1 Ma (MSWD = 1.14), 274.8 ± 1.8 (MSWD = 0.46) and 275.6 ± 0.7 (MSWD = 1.04), respectively. The whole-rock geochemistry of the four intrusions is characterized by REE and HFSE enrichment, Ba, Sr, P, Ti, and Eu depletion, εNd (t) values of – 1.2 to +0.3, two-stage Nd model ages (T2DM) of 1.14–1.02 Ga, and an A1-type granite affinity. Whole-rock zircon geothermometry indicates a high formation temperature of 721–924°C. It is concluded that these A-type granites were generated by partial melting of Neoproterozoic OIB-like rocks, coupled with extensive fractional crystallization and minor crustal contamination. The production of the A-type granites at ca. 275 Ma represented the termination and greatest degree of lithospheric thinning during large igneous province formation.

Erratum to: On the petrogenesis and evolution of U-rich granite: insights from mineral chemistry studies of Gattar granite, North Eastern Desert, Egypt

In Egypt, the younger granites are essentially undeformed and emplaced at shallow levels with typical A-type chemical characteristics. Petrogenesis of A-type granites in Egypt may be more intricate, especially where magmatic fluids and hydrothermal solutions are concerned. Moreover, these post-orogenic granites are attracting increased economic interest because some plutons like Gattar granite are associated with high concentrations of ore metals such U and Mo, and most of such U-rich granites have common features of localization of the U-mineralization within the alteration zones along sheared and fractured parts of the granites and intimate fluorite association. Our study demonstrated that the feldspars formed in a temperature below 700 °C and at a shallow depth, the temperature of transformation of zircon is high between 400 and 700 °C and shifted to less than 600 °C by an increasing F content in final differentiation products. Muscovite of Gattar granite is late to post-magmatic and formed in a temperature between 300 and 400 °C by hydrolysis of K-feldspars, and chlorite originated in nearly 300 °C on the average estimation. The chemical compositions of apatite reported that the apatite classified as fluoro-apatite with average F contents 3.56 % reflecting either primary magma conditions or subsequent metamorphism in the presence of relatively high F fluids. Pitchblende appeared in average 230 °C, and secondary uranium minerals emerged in average 150 °C. It is markedly noticeable that there is temperature gradation from very high temperature alteration near to solidification of granitic magma to very low alteration that represents prevalence of secondary uranium deposits. Our findings articulated the role of H2O- and F-bearing fluids in alteration of essential and accessory minerals and transformation of uranium element from solution and fluids into the crystal lattice of zircon as U+4 in an early stage event. Perhaps, the cause of this gradation is the evolution of U-rich granitic magma into residual fluids through magmatic differentiation and, afterwards, hydrothermal solutions through fluid-rock interaction.

Geochronological and geochemical constraints on the origin of the 304 ± 5 Ma Karamay A-type granites from West Junggar, Northwest China: implications for understanding the Central Asian Orogenic Belt

U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean 206Pb/238U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO2, FeOt/MgO, total alkalies (K2O + Na2O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P2O5, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial 87Sr/86Sr = 0.7014–0.7022, ϵNd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.

Age and petrogenesis of A-type granites in the middle segment of the Bangonghu–Nujiang suture, Tibetan plateau

During geological investigations conducted over the past several years, the authors found a group of A-type granites in the middle segment of the Bangonghu–Nujiang suture on the Tibetan plateau. These granitic plutons occur as stocks with exposed areas of less than 1km2 and lithologically include biotite granite and granodiorite porphyry, which had intruded into Cretaceous sedimentary rocks. Petrogeochemically, the A-type granites contain high SiO2 levels varying from 68.62%–75.36wt.%. Moreover, they contain relatively high Fe2O3T (0.86–5.39wt.%) and low Al2O3 (12.76–15.54wt.%) levels and are weak peraluminous to metaluminous. A trace element N-MORB-normalized spider diagram showed apparent enrichment of the large ion lithophile elements (LILE) Rb, Th, U, K, and Pb with marked depletions of Ba and Sr. The high field strength elements (HFSE) Nb, Ta and Ti were sharply depleted, with relative enrichment of Zr and Hf. These signatures are characteristic of A-type granite. The granite samples had high rare earth element (REE) contents with ∑REE=122.37–291.19ppm. Light REE were rich relative to heavy REE with LREE/HREE=4.89–9.58. And negative Eu anomalies were sharp with δEu varying from 0.14–0.5 and a mean value of 0.34. Their chondrite-normalized rare earth element distribution patterns appear as gently right-dipping V-types.Zircon U–Pb LAICPMS dating for three granitic plutons yielded weighted mean ages of 109.6±1.4Ma (MSWD=3.7), 112.3±0.51Ma (MSWD=0.21) and 113.73±0.5Ma (MSWD=0.21), indicating that they formed in the late stage of the early Cretaceous epoch. The Nd, Sr and Pb isotopic results showed that the granites were rich in radiogenic isotopes. The 87Sr/86Sr and ISr ratios varied from 0.719842–0.786395 and 0.706900–0.710378, respectively. The 143Nd/144Nd ratios were from 0.512123–0.512392, with relatively large negative εNd values (−3.37 to −10.34). The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranged from 18.703–19.070, 15.680–15.732 and 39.121–39.576, respectively, exhibiting isotopic signatures typical of crustal sources. All of the data demonstrated unanimously that these granites were derived from partial melting of arc basaltic rocks underplated in the lower crust during oceanic crust subduction. The high-temperature heat source for melting of anhydrous basaltic rocks of the lower crust was from upwelling asthenosphere through slab windows formed as a result of slab break-off during post-collisional extension of the orogenic zone.

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO 2, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ 18O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ 18O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron ( n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes). The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization (∼ 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their 18O/ 16O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ 18O (Fsp) values point to magmatic origin of fluids. The stable and radiogenic isotope data [ δ 18O (Zrn) = 5.82 ± 0.06‰, I Sr = 0.7022 ± 0.0064, ε Nd ( T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.

Oxidized, magnetite-series, rapakivi-type granites of Carajás, Brazil: Implications for classification and petrogenesis of A-type granites

The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO t/(FeO t + MgO), TiO 2/MgO, and K 2O/Na 2O and low Al 2O 3 and CaO compared to calc-alkaline granites. The contrast of Al 2O 3 contents in these two granite groups is remarkable. The CaO/(FeO t + MgO + TiO 2) vs. CaO + Al 2O 3 and CaO/(FeO t + MgO + TiO 2) vs. Al 2O 3 diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO t/(FeO t + MgO) and the FeO t/(FeO t + MgO) vs. Al 2O 3 and FeO t/(FeO t + MgO) vs. Al 2O 3/(K 2O/Na 2O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO 2 conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed.

A hybrid origin for the Qianshan A-type granite, northeast China: Geochemical and Sr–Nd–Hf isotopic evidence

Major and trace element, whole rock Sr and Nd isotope and zircon Hf isotope data are reported for a suite of A-type granites and mafic microgranular enclaves from the Early Cretaceous (126 ± 2 Ma) Qianshan pluton, Liaodong Peninsula, northeast China, with the aim of investigating the sources and petrogenesis of A-type granites. The Qianshan pluton includes hornblende alkali-feldspar granite, graphic biotite granite and mafic microgranular enclaves. The hornblende alkali-feldspar granites have high SiO 2, Fe 2O 3T / MgO, K 2O + Na 2O, Rb, Zr and LREE contents and low Ba and Sr concentrations with strongly negative Eu anomalies. Their high Rb / Sr ( 87Rb / 86Sr = 16.76–24.15) and initial 87Sr / 86Sr ratios (0.7215 to 0.7283), negative ε Nd( t) values (− 14.1 to − 16.5) and zircon ε Hf( t) values (− 18.9 to − 11.5) indicate they were mainly derived from a crustal source, but with involvement of high ε Nd( t) and ε Hf( t) materials. Graphic biotite granites have similar geochemical features and Sr–Nd–Hf isotopic compositions to enclaves, indicating they were the result of crystal fractionation of evolved mafic magmas, but with involvement of low ε Nd( t) and ε Hf( t) materials. The mafic enclaves have an igneous texture and contain acicular apatite, suggesting quenching of mafic magmas that have co-mingled with the host granites. They have low initial 87Sr / 86Sr ratios (0.7097–0.7148), negative ε Nd( t) (− 14.5 to − 11.9) and zircon ε Hf( t) (− 17.1 to − 6.9) values, and are enriched in LILEs and LREEs and depleted in HFSEs. When coupled with the high MgO (Mg# up to 54), this indicates derivation from an enriched lithospheric mantle source, but contaminated by crustal materials. Geochemical and Sr-, Nd- and zircon Hf-isotopic compositions rule out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mantle-derived mafic and crustal-derived magmas, coupled with crystal fractionation, is compatible with the data. This example shows that at least some A-type granites formed through a complex process involving mantle- and crustal-derived magma mixing, crystal fractionation and infracrustal melting.

Petrogenesis of Archaean Malene supracrustal rocks, NW Buksefjorden region, West Greenland: geochemical evidence for highly evolved Archaean crust

Abstract We report new data for major, minor and trace elements in forty-nine samples of Malene supracrustal rocks, NW Buksefjorden region, West Greenland. The samples consist principally of quartz±cordierite±biotite±garnet±anthophyllite/gedrite±sillimanite, with abundant accessory zircon, and minor rutile, allanite and monazite; plagioclase is rare to absent, while muscovite occurs in only a few cases. Thus, the Malene paragneisses closely resemble cordierite-orthoamphibole rocks found in other metamorphic complexes. The NW Buksefjorden Malene paragneisses have, on average, high SiO2, moderate Al2O3, Fe2O3(T) and MgO, and low K2O, Na2O and (especially) CaO. Distinctive trace element characteristics include: (1) low Sc, Co, Cr, Ni and Sr; (2) high Y, Zr, Nb, Hf, Ta, Th, U and REE, with prominent negative Eu-anomalies in nearly every case; and (3) high Ga (up to 65 ppm), with Ga Al ratios significantly higher than average crustal material. On the basis of AFM assemblage type, the paragneisses can be divided into distinct groups which preserve the field distinctions noted by previous workers. However, these groupings primarily reflect variations in mg (total range from 0.26–0.89), with no meaningful distinctions appearing among trace elements except possibly Cu. The trace element characteristics of the NW Buksefjorden Malene paragneisses are identical to those of the Malene quartz-cordierite gneisses found throughout the greater Godthabsfjord region. These gneisses are different from the Malene quartzite-psammite-pelite group of clastic metasediments. The trace element data of the quartz-cordierite gneisses are compatible with a protolith dominated by material derived from felsic igneous precursors, an interpretation supported by immobile element discriminant plots and ternary La-Th-Sc and Th-Co-Hf plots. However, negative Eu-anomalies and low Sr and Ca suggest that plagioclase was lost from the protolith or from the precursor lithology (or both). The high concentrations of Ga indicate either that the source rocks for the paragneisses were unusually Ga-rich, or that assembly of the protolith was accompanied by a major fractionation of Al from Ga. We propose that the protolith for most of the NW Buksefjorden Malene paragneisses was a quartz-chlorite-sericite schist derived from felsic volcaniclastic sediment hydrothermally altered by seawater prior to metamorphism. This alteration resulted in gain of Mg (and possibly Fe) and loss of lime and alkalis (and possibly Sr and Eu). This hypothesis contrasts with earlier proposals that the protolith included Mg-rich clay (palygorskite) that precipitated directly from seawater, but it is more consistent with the new data. There is a remarkably close correspondence between the element enrichments (and depletions) shown by the Malene paragneisses and those found in A-type granitic rocks and their volcanic equivalents. Consequently, we suggest a comparable source for the paragneisses. The petrogenesis of A-type granites is associated typically with thickened, stable continental crust. This implies that the protolith for the Malene paragneisses formed after, or in conjunction with, a major event of Archaean crustal thickening.