Petrogenesis Of Granites Research Articles

Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland

Dehydration melting of metasupracrustal rocks at mid- to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ∼1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850 °C), at 20%–30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000–1 050 °C, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.

Petrogenesis of the Early Paleozoic S-type granites in southern Alxa, Northwest China and its implications on fluid flourishing in the middle and lower crust

Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO2 (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO2 (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO2 = 72.50–76.04 wt%) are different from the two-mica granites in having higher Na2O (3.38–4.58 wt%) and lower K2O (3.70–4.90 wt%) with higher Na2O/K2O ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.

Petrogenesis of the Pandao granites in the Wutai Mountains area in the North China Craton: Constraints from geochemistry, zircon U–Pb geochronology, and Hf isotopes

The Trans‐North China Orogen (TNCO) is a part of the North China Craton (NCC) and together provides a classic example of lithospheric destruction. The Pandao granites outcropping in the Wutai Mountains area provide a window to investigate the Mesozoic magmatism in the TNCO. Here, this paper presents the new zircon geochronology, whole‐rock geochemistry, and zircon Hf isotope data for the Pandao granites to discuss their petrogenesis and tectonic implications. The Pandao granites are mainly composed of light‐red medium‐ to coarse‐grained biotite granite and light‐grey fine‐ to medium‐grained biotite granite. Zircon U–Pb ages of 110.05 ± 0.67 Ma and 108.35 ± 0.81 Ma suggest that the Pandao granites were crystallized in the Early Cretaceous. The Pandao granites are classified as high‐K calc‐alkaline and weak peraluminous series. The rocks display abundance in large‐ion lithophile elements (LILE) and light rare‐earth elements (LREE) but show depletion in high‐field‐strength elements (HFSE) and heavy rare‐earth elements (HREE), with strong negative Eu anomalies. The classification diagrams indicate that the Pandao granites are A‐type granites and thus belong to the A1 subtype, formed in an intraplate extensional environment. The Pandao granites have homogeneous zircon Hf isotopic compositions. Their zircons have negative εHf(t) values (−19.1 to −17.1) and old Hf isotope crustal model ages (2,250–2,375 Ma), suggesting that the Pandao granites were formed by partial melting of the Paleoproterozoic lower crustal material. Therefore, it is suggested that the Pandao granites were formed under an intraplate extensional tectonic environment of remote effect due to the ancient Pacific Plate subduction and retreat beneath the Eurasian continent. The TNCO was influenced by the subduction and retreat of the ancient Pacific Plate in the late Early Cretaceous.

Petrogenesis of the early Paleoproterozoic low-δ18O potassic granites in the southern NCC and its possible implications for no confluence of glaciations and magmatic shutdown at ca. 2.3 Ga

The interval of 2.45 to 2.20 Ga has previously been interpreted to contain a plate tectonic shutdown, but newly discovered magmatism of this stage is filling in the early Paleoproterozoic magmatic gap. However, the crust evolutionary scenarios and geodynamic driving mechanisms during this stage are still lacking comprehensive understanding. The present study verifies that the igneous rocks formed during this interval did occur along the southern margin of the NCC. The high-K granites in the Xiong’ershan area are dated at 2.32 to 2.29 Ga. Their low-δ18O values of zircons (-0.92 to + 4.78‰, 3.26‰ on average) are reported for the first time, representing the primary record of low-δ18O magma in the early Paleoproterozoic. The high-K granites show transitional features between S-type and normal A- and I-type granites. Zircon εHf(t) (-7.6 to + 1.5) and whole-rock εNd(t) values (-3.8 to + 1.1) with much older TDM2 (Hf TDM2 = 3.36 to 2.77 Ga and Nd TDM2 = 3.08 to 2.63 Ga, respectively) than their formation ages, together with heterogeneous major-element composition point to recycling of continental crust older than ca. 2.77 Ga. Low CaO/(FeOt + MgO), low to moderate Al2O3/TiO2 (36.7 to 292), Rb/Sr (0.51 to 5.67) and Rb/Ba (0.04 to 1.09), variable but mostly high CaO/Na2O (0.05 to 5.17) ratios commonly imply metapsammite-dominated (meta-greywacke) sources with minor pelite-derived and tonalites materials. Considering that zircons lack metamict features and have low but positive andhomogeneous δ18O values, we prefer a high-temperature hydrothermally altered older crustal source by surface water before or during partial melting. The great diversity of rock assemblages and geochemical features require a thermal anomaly linked to an extensional setting to account for the crustal melting. The occurrence of potassic rocks, and spatially associated gabbros, diorites and TTGs likely indicate no confluence of the Paleoproterozoic magmatic gap and glaciations during the 2.35 to 2.29 Ga at least in the southern NCC.

The production of granitic magmas through crustal anatexis at convergent plate boundaries

Granite petrogenesis is associated with the formation and evolution of continental crust along convergent plate boundaries. While magmatic processes such as partial melting and fractional crystallization result in the compositional diversity of granites, the nature of magma sources is related to tectonic settings of granite generation. Although granites consist predominantly of magmatic minerals, they generally contain small amounts of peritectic and residual minerals. This results in the compositional difference between natural granites and experimental melts. In addition to the fractional crystallization of mantle-derived mafic magmas for the generation of granites during accretionary orogeny, a wide range of crustal rocks can be partially melted to form granites in both accretionary and collisional orogens. In the latter case, metasedimentary and metaigneous crustal sources lead to the dichotomy of S- and I-type granites. Geochemically, granites with negative εNd(t) and εHf(t) values are certainly derived from reworking of the ancient crust. On the other hand, granites with positive εNd(t) and εHf(t) values may be produced either by reworking of the juvenile crust or by fractional crystallization of asthenospheric mantle-derived magmas at actively convergent plate boundaries. In this case, major and trace elements may be of use in distinguishing between the crustal and mantle contributions. Granite, migmatite and granulite generally form lithological associations at previously converged plate boundaries, with migmatitic domes as the major component of metamorphic core complexes. Their formation is related to extensional heating subsequent to thinning of the thickened mantle lithosphere. As soon as the thickened lithospheric mantle was foundered into the asthenospheric mantle, it makes the space for asthenospheric upwelling to induce active rifting along the thinned lithosphere. This provides an external heat source for anatectic metamorphism at high thermal gradients, marking the tectonic transition in the property of orogenic systems into the rifting stage. Therefore, the active rifting is spatially and temporally dictated by thinning of the orogenic mantle lithosphere, and its operation is a geodynamic mechanism for crustal anatexis at convergent plate boundaries. Consequently, the lithospheric thickness has a profound effect on crustal anatexis for granitic magmatism, which is considerably later than plate convergence.

Petrogenesis of biotite granite with transitional I-A-type affinities: Implications for continental crust generation

The I-S-A-type classification scheme for granites has been widely used to identify potential sources and the tectonic setting of granitic magmatism. Recent geochemical studies have shown that most granites are derived from a mixture of mantle and crustal sources, and that granitic magmatism contribute to generation and differentiation of the continental crust. However, the origin of granites with transitional affinities remains unclear, along with their role in the growth and differentiation of the continental crust. In this contribution, the ca. 91 Ma Bozhushan biotite granites in the southwestern Cathaysia Block, China, are characterized by high abundance of high-field-strength elements (HFSEs), with Zr + Nb + Ce + Y > 350 ppm and 10000*Ga/Al > 2.6, which are features typical of A-type granites. However, biotites in the Bozhushan granites are compositionally distinct from those of typical A- and S-type granites. Decreasing P2O5 contents with increasing SiO2 contents are consistent with an I-type rather than a S-type granite. Petrographic observations and geochemical data suggest that the Bozhushan granites have transitional I-A-type affinities. The Bozhushan granites contain remarkably evolved isotopic compositions, with εNd(t) = −9.2 to −8.0 and εHf(T) = −9.8 to −7.3, which are slightly lower than values for Late Cretaceous Baiyunshan alkaline igneous rocks. The Bozhushan granites have the low Y/Nb and Yb/Ta ratios, which are similar to the Baiyunshan alkaline igneous rocks, and continental alkaline basalts from the East African Rift and eastern China, ocean island basalts, and A1-type granitoids. Geochemical data indicate that the Bozhushan granites were likely generated by high-temperature melting of a hybrid source containing underplated alkaline igneous rocks and Precambrian basement rocks. Although the Bozhushan granites have crustal isotopic compositions, our data indicate that these granites also represent the formation of new crust. Thus, granites with transitional I-A-type affinities, such as the Bozhushan granite, have a role in the generation of continental crust.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt <1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.

Petrogenesis of early Eocene granites and associated mafic enclaves in the Gangdese batholith, Tibet: Implications for net crustal growth in collision zones

Granitoids and related mafic microgranular enclaves (MMEs) in continental collision zones may provide crucial insights into the formation and evolution of continental crust. In this paper, we report an investigation into the mineralogy, zircon U–Pb geochronology, whole-rock major and trace elements, and Sr–Nd–Hf isotopic geochemistry of granodiorites and hosted MMEs, as well as monzogranites, from the Zhaxiding intrusive complex in the Gangdese batholith, southern Tibet. Zircon U–Pb dating suggests that these rocks were emplaced during the early Eocene (49 to 47 Ma). Field investigation and petrological observations indicate that the MMEs represent globules of a mafic magma that was injected into the host felsic magma. On the basis of depleted zircon Hf (εHf(t) values of +6.5 to +9.2) and whole-rock Sr–Nd (εNd(t) values of +1.6 to +2.6) isotopic compositions, along with their whole-rock compositions, the MMEs are interpreted as having resulted from mixing of mantle-derived magma with juvenile crust-derived melts. The granodiorites and monzogranites are metaluminous and have positive zircon εHf(t) (+6.3 to +8.9 for granodiorites and +1.7 to +10.2 for monzogranites) and whole-rock εNd(t) (+1.6 to +1.9 for granodiorites and +0.5 to +1.1 for monzogranites) values. The granodiorites further exhibit continental crust-like chemical compositions (e.g., enrichment in Rb, K, and Pb and depletion in Nb and Ta) and high Mg# values, suggesting that they originated from partial melting of juvenile crust with the involvement of mantle-derived melts. However, the monzogranites have lower P, Ti, and total rare earth element contents, similar Rb, Ba, and Sr contents, and uniform Eu anomalies compared with the granodiorites, and were likely generated by fractional crystallization of amphibole and Fe–Ti oxides from juvenile crust-derived magma. We propose that underplating of mantle-derived magma likely triggered coeval crustal magmatism and that mixing of the mantle- and crust-derived magmas and subsequent crystallization differentiation formed the intermediate to felsic Zhaxiding intrusive complex in the Gangdese batholith. The continental crust-like bulk compositions of the intermediate to felsic magmatic rocks represent a net addition of juvenile material to continental crust. Our study thus indicates that magma mixing and differentiation play a significant role in the formation of andesitic to dacitic continental crust in collision zones.

Petrogenesis of Archaean granites in the Barberton region of South Africa as a guide to early crustal evolution

ABSTRACT The Barberton region of South Africa is characterized by a broad variety of granite types that range in age from ca. 3.5 Ga to 2.7 Ga and reflect the processes involved in the formation of Archaean continental crust on the Kaapvaal Craton. These granites are subdivided into three groups, as follows: A tonalite-trondhjemite-granodiorite (TTG) suite diapirically emplaced at 3 450 Ma and 3 250 Ma into pre-existing metamorphosed greenstone belt material. TTG melts were derived from melting amphibolite in the lower crust, with individual plutons being emplaced at various crustal levels. The dome-and-keel geometry that characterizes the TTG-greenstone dominated crust at this time is inconsistent with a plate tectonic domain and reworking was likely controlled by gravity inversion or ‘sagduction’; Regionally extensive potassic batholiths (the GMS suite) were emplaced at 3 110 Ma during a period of crustal thickening and melting of a TTG-dominated lower crust. Subsequent to emplacement of the voluminous GMS granites, the thickened continental crust had stabilized sufficiently for large sedimentary basins to form; Late granite plutons were emplaced along two distinct linear and sub-parallel arrays close to what might have been the edge of a Kaapvaal continent at 2 800 to 2 700 Ma. They are subdivided into high-Ca and low-Ca granites that resemble the I- and S-type granites of younger orogenic episodes. The high-Ca granites are consistent with derivation from older granitoids in the lower crust, whereas the low-Ca granites may have been derived by melting metasedimentary precursors in the lower-mid crust. Granites with similar characteristics are associated with a subduction zone in younger terranes, although the recognition of such a feature at Barberton remains unclear. The petrogenesis of granites in the Barberton region between 3.5 Ga and 2.7 Ga provides a record of the processes of Archaean crustal evolution and contributes to discussions related to the onset of plate tectonics.

Petrogenesis and tectonic setting of the early-middle triassic subduction-related granite in the eastern segment of East Kunlun: evidences from petrology, geochemistry, and zircon U-Pb-Hf isotopes

ABSTRACT The subduction process of the Paleo-Tethyan Ocean is still under controversy. We report zircon U-Pb ages and geochemistry (major and trace elements, and Hf isotopic compositions) for the Xiangjiananshan granitic plutons in the eastern segment of the East Kunlun Orogenic Belt, northern Tibetan Plateau, to constrain the subduction-related granitoids in its petrogenesis and to reconstruct the subduction prosess of the Buqingshan-A’nyemaqing Ocean. Zircon U-Pb dating yields coeval ages of 246.6 ± 4.1 Ma for granodiorite and 246.4 ± 3.9 Ma for biotite monzogranite of the Xiangjiananshan granitoid pluton. The granitoids include granodiorites, biotite monzogranites, and pseudo-porphyritic monzogranites. The granitoids have high contents of SiO2 (66.60–73.19 wt.%) and Al2O3 (13.28–16.38 wt.%), consistent with the high-K calc-alkaline series. The A/CNK ratios range from 0.98 to 1.02, indicating that the granodiorite is metaluminous and weakly peraluminous. The mafic microgranular enclaves (MMEs) in granodiorite and biotite monzogranite have a restricted SiO2 range (57.24–60.03 wt.%), and have higher Fe2O3 T, and MgO contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. All samples are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and are enriched in large ion lithophile elements (LILEs; e.g., Rb, Nd and La) and light rare earth elements (LREEs). Zircons from the pseudoporphyritic monzogranite yield two-stage model ages of 1137.32 to 1588.12 Ma, together with εHf(t) values from −5.01 to +2.11. These results indicate that the parental magma of pseudoporphyritic monzogranite was generated by partial melting of Paleoproterozoic to Mesoproterozoic rocks of the lower crust with contamination of ancient crustal material. Field and petrological observations, together with elemental and Hf isotopic data, indicate that the Xiangjiananshan granitic plutons was formed by magma mixing of mafic and felsic magmas. The felsic magma was derived from partial melting of the lower crust, while the mafic magma was derived from partial melting of an enriched lithospheric mantle metasomatized by slab-derived fluid due to the subduction of the Buqingshan-A’nyemaqing Ocean. Therefore, we propose that the northward subduction of the Buqingshan-A’nyemaqing ocean north to the Bayan Hara block under East Kunlun initiated in the Middle Permian (ca. 270 Ma), yielding calcium metaluminous arc magmatic rocks with I-type granite characteristics at East Kunlun, including the XG pluton in this study. Subsequently, the Buqingshan-A’nyemaqing ocean closed and resulted in collision of the Bayan Hara and East Kunlun blocks from 240 to 230 Ma.

Petrogenesis of ore-forming granites with implications for W mineralization in the Huangsha-Tieshanlong Deposit, Southern Jiangxi Province, South China

Mesozoic magmatism and associated mineralization are widespread in the South China Block (SCB). Tungsten-polymetallic deposits in the Nanling Range within the central SCB represent one of the most important tungsten regions in the world. The Huangsha-Tieshanlong W deposit is an important large quartz vein-type W-polymetallic deposit in southern Jiangxi Province, in the eastern Nanling Range. Zircon UPb analyses of six representative samples of the ore-related granitoids from a tunnel in the Huangsha-Tieshanlong W deposit yielded ages of 152.2 ± 3.8 Ma, 154.0 ± 2.8 Ma, 150.4 ± 4.6 Ma, 151.3 ± 2.7 Ma, 141.6 ± 4.2 Ma, and 142.1 ± 4.6 Ma. These granitoids have low εHf(t) values (−15.7 to −7.6), and old two-stage model ages (1746–1347 Ma), indicating that they were derived from the partial melting of Proterozoic crustal materials. These ages combined with other geochronological data in the region suggest that the Huangsha-Tieshanlong pluton was formed by at least three stages of granitic magmatism during the Middle Jurassic (~168 Ma), Late Jurassic (~152 Ma), and Early Cretaceous (~142 Ma), which coincides with the formation ages of the WSn deposits at 171–160 Ma, 159–150 Ma, and 149–144 Ma, respectively. Thus, the intrusion of the granite and the associated WSn mineralization in the Huangsha-Tieshanlong deposit occurred almost synchronously. It is worth noting that our study indicates that the Early Cretaceous (~142 Ma) is probably another important stage of granitic magmatism and mineralization in the eastern Nanling region.

Apatite geochemical and Sr[sbnd]Nd isotopic insights into granitoid petrogenesis

Apatite geochemical and SrNd isotopic compositions provide robust means for constraining a wide range of geological processes in earth and planetary sciences, including magmatism, metamorphism, metasomatism, planetary geochemistry, and geochronology. Here we extend the utility of these data to fingerprint magmatic evolutionary processes and granitoid petrogenesis. Apatite grains in Early Cretaceous (ca. 130 Ma) granite from northeastern China have distinct occurrences, geochemical features, and SrNd isotopic compositions, indicating their different origins. Geochemically, they can be divided into two groups as follows. Group 1 apatites mainly occur as euhedral to subhedral crystals, and have relatively low initial 87Sr/86Sr ratios (0.7071–0.7110), weakly negative εNd(t) values (−11.5 to −4.9), and variable chemical features such as positive to negative Eu anomalies and continuously variable total rare-earth element, Sr, Y, and Th contents, suggesting crystallization from host magma. Combining with petrographic observations and whole-rock geochemistry, it is indicated that Group 1 magmatic apatite records the history of magma evolution, including the crystallization history of minerals, and possibly fractionation of early crystallized minerals such as plagioclase and titanite. Group 2 apatites occur as anhedral crystals, and display variable initial 87Sr/86Sr ratios (0.7010–0.7506) and strongly negative εNd(t) values (−33.4 to −7.7) at the emplacement age of the host granite, suggesting they did not crystallize from the magma, nor were they inherited from its source. Group 2 apatites have geochemical and SrNd isotopic compositions comparable with those from local Archean and Paleoproterozoic intrusive rocks, suggesting capture from wall rocks and interaction with the host melt. The occurrence of assimilated apatite grains in the host granite suggests a crustal assimilation process involved in granite petrogenesis, which is not easily identified through whole-rock geochemistry. Our results confirm that apatite in situ geochemical and SrNd isotope analyses are powerful tools for elucidating the source and evolution of magma, thus providing compelling evidence for the origin of granitoids.

Mixing of cogenetic magmas in the Cretaceous Zhangzhou calc-alkaline granite from southeast China recorded by in-situ apatite geochemistry

AbstractMixing of cogenetic magmas represents an important process in granite petrogenesis but is difficult to identify and is consequently often overlooked due to the absence of obvious isotopic distinctions between the mixed melts. We have conducted in situ elemental and O isotope analyses on apatite from Cretaceous Zhangzhou calc-alkaline granite in southeast China. We integrated these data with micro-analyses on other minerals (plagioclase, zircon, and titanite) as well as whole-rock geochemistry to decipher the mixing history of this granitic complex. The apatite occurs as an early crystallizing phase forming inclusions in biotite, plagioclase, and titanite, and is characterized by core-rim zonation textures with a dark core and bright rims in backscattered images. The core domains have remarkably higher SO3 and Li concentrations but much lower SiO2, REE, and Y concentrations than the rim domains. However, both the cores and rims show geochemical compositions similar to that from typical I-type granite and also have mantle-like O isotope compositions (the core has δ18O = 5.3–6.8‰ and the rim has δ18O = 5.2–6.4‰, respectively), indicating crystallization from granitic melts derived from newly accreted crust. The combined major and trace element and O isotope compositions of apatite and whole-rock geochemistry suggest that compositional evolution of the Zhangzhou granite involved mixing between two cogenetic magma batches, with variable degrees of subsequent differentiation. Batch I magma was a low-SiO2 and high-SO3 melt, whereas Batch II magma was a high-SiO2 and low-SO3 melt that experienced devolatilization. The high-S content in apatite cores further suggests the parental magma of the Zhangzhou granite likely originated from a sulfur-rich source comprising mainly newly accreted arc crust in response to subduction of the paleo-Pacific Ocean. The geochemical records of these magmatic processes are rarely observed in coeval zircon, titanite, and plagioclase. Our study, therefore, demonstrates that apatite geochemistry is potentially a more suitable monitor of complex magmatic evolution, including devolatilization and mixing of isotopically indistinguishable magmas.

Petrogenesis of late-Paleoproterozoic A-type granites in the Tarim Craton and implications for the Columbia assembly and break-up

塔里木克拉通保存有古元古代晚期与哥伦比亚超大陆演化相关的岩浆和构造记录。本文综述了塔里木克拉通周缘的古元古代晚期A型花岗岩，识别出两期（1.95Ga和1.85~1.73Ga）具不同地球化学特征的A型花岗岩。两期花岗岩均来源于具英云闪长岩和花岗闪长岩成分特征的地壳的部分熔融。~1.95Ga花岗岩具有相对较高的SiO₂和Sr含量、Y/Nb和Ce/Nb比值、较低的ε_Hf（t）值（-13~-5.2），为A2型花岗岩，形成于活动大陆边缘或弧后伸展等构造环境；而1.85~1.73Ga花岗岩具有相对较低的SiO₂和Sr含量、Y/Nb和Ce/Nb比值、和较高的ε_Hf（t）（-5.9~+8.7）和ε_Nd（t）（-6.2~-2.5）值，为A2向A1过渡类型，形成于碰撞后伸展或者陆内伸展背景。笔者综述还发现塔里木克拉通内部元古代晚期岩浆和变质也可以明显的分为两期，岩浆岩峰值分别为1.93Ga和1.85Ga，变质年龄峰值为1.96Ga和1.84Ga，说明塔里木克拉通内部存在两期不同的古元古代晚期构造-岩浆事件。这两期构造-岩浆事件对应于俯冲型造山和碰撞造山后伸展环境，可能分别记录了哥伦比亚超大陆聚合和裂解过程，也就是说哥伦比亚超大陆可能在1.85Ga左右已经开始部分裂解。;Lots of late-Paleoproterozoic magmatic and tectonic records, which are commonly interpreted as products of the Columbia assembly and break-up, were preserved in the Tarim Craton. In this study, we summarized the late-Paleoproterozoic A-type granitoids at the periphery of the Tarim Craton, and identified two A-type granitoid flare-ups at 1.95Ga and 1.85~1.73Ga. All these A-type granitoids are derived from partial melting of the crustal materials with tonalitic to granodioritic compositions. The ca. 1.95Ga granites show high SiO₂ and Sr contents, high Y/Nb and Ce/Nb ratios, low ε_Hf(t) values (-13~-5.2), and could be classified as A2 subtype granitoid. They were formed in continental arc or back-arc settings. The 1.85~1.73Ga granites show relatively low SiO₂ and Sr contents, low Y/Nb and Ce/Nb ratios, high ε_Hf(t) (-5.9~+8.7) and ε_Nd(t) (-6.2~-2.5) values, and could be classified as A2 to A1 transitional subtype. They were formed in post-collisional or intracontinental extension settings. Besides, magmatism and metamorphism ages in the Tarim Craton also show two peaks at 1.93Ga and 1.85Ga, 1.95Ga and 1.84Ga, respectively. These age data suggest that the Tarim Craton may have experienced two significant tectono-magmatic events at about 1.93Ga and 1.85Ga. These two events may record the subduction related orogeny and collisional orogeny, and are related to the Columbia assembly and break-up, respectively. We believed that the Columbia supercontinent may have broken up at 1.85Ga.

Geochemical and thermodynamic modeling of the petrogenesis of A1-type granites and associated intermediate rocks: A case study from the central Fennoscandian Shield

The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.

Whole-rock and biotite geochemistry of granites from the Miao’ershan batholith, South China: Implications for the sources of granite-hosted uranium ores

Granite-hosted uranium deposits are the most important types of uranium deposits in South China, but the metal sources of these deposits are still poorly constrained. The Miao’ershan batholith (South China) is a natural laboratory to address this issue, as well as to highlight the use of biotite composition as an indicator for the petrogenesis and uranium fertility of granites. Here, we present whole-rock and biotite geochemical data for U-bearing (Zhangjia and Douzhashan) and barren (Xiangcaoping and Paleozoic) granites of the Miao’ershan batholith. Contents of Li, Be, Mn, Sn, In, Nb, Ta, Ga, W, and U in the studied biotites increase from barren to U-bearing granites, while Mg, Ni, Co, V, and Rb display the opposite trend, which may indicate higher degrees of fractionation of the U-bearing granites. An Fe/(Fe + Mg) ratio of 0.65 in biotite appears to be an efficient threshold to discriminate between U-bearing and barren granites. New ternary discrimination diagrams are introduced, based on Li–V × 20–Mn and Sn–Co × 5–Nb, respectively, to aid the recognition of the U-bearing and barren granites. The relatively high whole-rock U concentrations (8.0–38.6 ppm) and low whole-rock ratios of Th/U (0.5–3.2) and REE/U (2.1–13.5) of the Zhangjia and Douzhashan granites favored the crystallization of uraninite. The samples experiencing fluid-wallrock interaction from the Zhangjia granite are characterized by low whole-rock U concentrations of 3.0 to 4.2 ppm and high Th/U ratios of 8.2 to 10.1, suggesting that a significant part of the U has been leached out by oxidizing meteoric waters. This confirms that the granite represents the primary source of U at the Zhangjia uranium deposit.

Apatite geochemistry as an indicator of petrogenesis and uranium fertility of granites: A case study from the Zhuguangshan batholith, South China

To explore the potential of apatite geochemistry as an indicator of petrogenesis and uranium fertility of granites, we analyzed the major and trace elements of apatites from U-bearing (Changjiang and Youdong) and barren (Jiufeng and Fuxi) granites in the Zhuguangshan batholith, South China. All the investigated apatites are identified as fluorapatite that is typical for igneous apatite. Apatites from the U-bearing plutons have higher concentrations of Mn, Fe, Na, and Y, and lower Sr, (Eu/Eu*)N, and (La/Yb)N than those from the barren ones. The compositional variations are interpreted to be related to whole-rock parameters such as SiO2 abundance, oxidation state, and aluminum saturation index. In addition, the presence of other REE-Th-rich minerals such as monazite and allanite can affect REE and Th concentrations in apatites by competing for these elements during apatite crystallization. Concentrations of Sr, Y, and Mn in the studied apatites can serve as a proxy of whole-rock compositions. Apatite compositions indicate that magmas of the U-bearing plutons have higher degrees of magmatic differentiation and lower oxygen fugacity than those of the barren ones. Our results suggest that Sr, Mn, Y, Na, Fe, (La/Yb)N and (Eu/Eu*)N in apatites are effective in distinguishing U-bearing granites from the barren ones. This study highlights the potential of apatite geochemistry to assist in deciphering petrogenesis and uranium fertility of granites.

Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.

Silurian-Devonian granites and associated intermediate-mafic rocks along the eastern Kunlun Orogen, western China: Evidence for a prolonged post-collisional lithospheric extension

The eastern Kunlun Orogen hosts numerous granitoid intrusions and associated intermediate-mafic bodies, that bear significant information on the tectonic evolution of this orogenic belt and the genesis of many ore deposits (e.g., Xiarihamu NiCo sulfide deposit), but the timing and petrogenesis of these rocks remain not well understood. In this contribution, the petrogenesis of representative Silurian-Devonian granites and associated intermediate-mafic rocks at Wulonggou, Xiarihamu and Kayakedengtage has been investigated. Zircon UPb dating results reveal that these intrusions were emplaced over a period of ~50 million years (m.y.) from 427 Ma to 382 Ma. Syenogranites at Wulonggou and Xiarihamu as well as quartz monzonites and porphyritic monzogranites at Kayakedengtage have characters of metaluminous A2-type granites. Whole-rock SrNd isotope data indicate that Syenogranites at Wulonggou (427–414 Ma) and Xiarihamu (396–391 Ma) were probably derived from earlier calc-alkaline I-type diorite-granodiorite, whereas quartz monzonites (ca. 424 Ma) and porphyritic monzogranites (ca. 421 Ma) at Kayakedengtage were most likely produced by partial melting of Early Paleozoic granulites. Monzogranites (394 Ma) and granodiorites (397 Ma) at Kayakedengtage and dolerites (426 Ma) from Wulonggou are characterized by positive εNd(t) values, indicative of a depleted mantle origin. The formation of voluminous A2-type granites with mafic rocks from the asthenosphere since 427 Ma occurred immediately after the subduction, supporting a post-collisional setting and a slab break-off model. The extensive magmatism coupled with asthenospheric upwelling and rapid orogenic uplift suggests two large-scale extension events at 427–414 Ma and 403–380 Ma in the orogen. The 411–406 Ma Xiarihamu NiCo sulfide deposit formed during relatively weak lithospheric extension and asthenospheric upwelling between these two extensional events. These results together with 373–357 Ma metaluminous A2-type syenogranites and I-type granodiorites at Qimantagh suggest a prolonged (~70 m.y.) and episodic post-collisional extension in the orogen, which was controlled by cyclic asthenospheric upwelling.

Petrogenesis of Late Early Cretaceous high-silica granites from theBangong–Nujiang suture zone, Central Tibet

High-silica granites are common in collisional orogenic belts. However, the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated. In this study, we present zircon U–Pb ages and Hf isotopes, and whole-rock geochemical and Sr–Nd isotopic compositions for biotite granites from the Jiang Co area, in the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating reveals that the Jiang Co granites were emplaced during the late Early Cretaceous (ca.114 Ma). These granites are characterized by high and variable SiO2 (69.7–78.2 wt%) and K2O (4.7–8.4 wt%), and low MgO (0.09–1.25 wt%) and P2O5 (< 0.1 wt%) contents. The samples are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, Ti, and Eu. These geochemical features suggest that the Jiang Co granites underwent extensive fractional crystallization. The high and variable initial 87Sr/86Sr (0.7066 to 0.7095) ratios and consistent εNd(t) (–8.2 to –9.2) values, along with TNd2DM ages of 1.66 to 1.58 Ga, and negative to positive zircon εHf(t) (−7.5 to +1.8) values, imply that these granites were formed by partial melting of the mature ancient crustal materials with minor involvement of mantle materials. Along with contemporaneous magmatic activity, we suggest that the Jiang Co granites were mainly generated by crustal melting in a post-collisional setting resulting from slab breakoff after the Lhasa–Qiangtang collision, and subsequently underwent a high degree fractional crystallization.