Assimilation And Fractional Crystallization Research Articles

Post-collisional potassic magmatism in the eastern Lhasa terrane, South Tibet: Products of partial melting of mélanges in a continental subduction channel

Post-collisional, potassic magmatic rocks widely distributed in the eastern Lhasa terrane provide significant information for comprehensive understanding of geodynamic processes of northward subduction of the Indian lithosphere and uplift of the Tibetan Plateau. A combined dataset of whole-rock major and trace elements, Sr–Nd–Pb isotopes, and in situ zircon U–Pb dating and Hf–O isotopic analyses are presented for the Yangying potassic volcanic rocks (YPVR) in the eastern part of the Lhasa terrane, South Tibet. These volcanic rocks consist of trachytes, which are characterized by high K2O (5.46–9.30wt.%), SiO2 (61.34–68.62wt.%) and Al2O3 (15.06–17.36wt.%), and relatively low MgO (0.47–2.80wt.%) and FeOt (1.70–4.90wt.%). Chondrite-normalized rare earth elements (REE) patterns display clearly negative Eu anomalies. Primitive mantle-normalized incompatible trace elements diagrams exhibit strong enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and display significantly negative Nb–Ta–Ti anomalies. Initial isotopic compositions indicate relatively radiogenic Sr [(87Sr/86Sr)i=0.711978–0.712090)] and unradiogenic Nd [(143Nd/144Nd)i=0.512121–0.512148]. Combined with their Pb isotopic compositions [(206Pb/204Pb)i=18.615–18.774, (207Pb/204Pb)i=15.708–15.793, (208Pb/204Pb)i=39.274–39.355)], these data are consistent with the involvement of component from subducted continental crustal sediment in their source region. The whole-rock Sr–Nd–Pb isotopic compositions exhibit linear trends between enriched mantle-derived mafic ultrapotassic magmas and relatively depleted crustal contaminants from the Lhasa terrane. The enrichment of the upper mantle below South Tibet is considered to result from the addition of components derived from subducted Indian continental crust to depleted MORB-source mantle during northward underthrusting of the Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ~55Ma. Secondary Ion Mass Spectrometry (SIMS) U–Pb zircon analyses yield the eruptive ages of 10.61±0.10Ma and 10.70±0.18Ma (weighted mean ages). Zircon Hf isotope compositions [ƐHf(t)=−4.79 to −0.17], combined with zircon O isotope ratios (5.51–7.22‰), imply an addition of crustal material in their petrogenesis. Clinopyroxene-liquid thermobarometer reveals pressure (2.5–4.1kbar) and temperature (1029.4–1082.9°C) of clinopyroxene crystallization, suggesting that depth of the magma chamber was 11.6–16.4km. Energy-constrained assimilation and fractional crystallization (EC–AFC) model calculation indicates depth of assimilation and fractional crystallization in the region of 14.40–18.75km underneath the Lhasa terrane, which is in consistent with depth of the magma chamber as suggested by clinopyroxene-liquid thermobarometer. Based on the whole-rock major and trace elements and Sr–Nd–Pb isotope compositions, combined with EC–AFC modeling simulations and zircon Hf–O isotope data, we propose that the YPVR resulted from assimilation and fractional crystallization (AFC) process of the K-rich mafic primitive magmas, which were caused by partial melting of the Indian continental subduction-induced mélange rocks.

Neoproterozoic active continental margin of the Cathaysia block: Evidence from geochronology, geochemistry, and Nd–Hf isotopes of igneous complexes

Previous studies have suggested that northwest Cathaysia was a passive continental margin during Neoproterozoic time, given the absence of Neoproterozoic magmatism. However, this view has been challenged by recent discoveries of mafic and felsic magmatic rocks with geochemical affinities to continental arc or back-arc volcanic rocks from the 1.0–0.9Ga Wuyi–Yunkai belt, indicating the development of an early Neoproterozoic continental margin arc and back-arc system in the Cathaysia block. In this paper we present new age data for the Miaohou and Shanhou arc complexes, demonstrating that the active continental margin of the Cathaysia block lasted until ca. 830Ma. The Miaohou complex is composed of gabbro, diorite, and granite, the Shanhou complex of diorite and granite, and both complexes are bound by the Jiangshao Fault. Twelve samples from the Miaohou and Shanhou complexes give zircon U–Pb ages of ca. 830Ma, indicating that the magmatism of the Miaohou and Shanhou complexes were coeval. The Miaohou gabbro has high zircon ɛHf(t) (+2.8 to +9.6) and whole-rock ɛNd(t) (+3.9) values together with the geochemical features of volcanic arc basalts, while the Miaohou and Shanhou diorites and granites show large variations and high values of zircon ɛHf(t) and ɛNd(t) [ɛNd(t) (+1.1 to +5.5) and ɛHf(t) (−6.1 to +12.3) for the diorites, and ɛNd(t) (+0.3 to +3.1) and ɛHf(t) (−3.8 to +11.3) for the granites] together with a similar geochemistry to continental arc felsic rocks (e.g., Cordilleran batholiths). The geochemical and isotopic data suggest that the southeastwards subduction of an oceanic plate under the Cathaysia block caused the dehydration of sediments in the subducted slab, leading to the metasomatism of the mantle wedge by subduction-related fluids, partial melting of the mantle wedge, followed by varying degrees of assimilation and fractional crystallization (AFC) as the magmas ascended. These processes provide a reasonable explanation for the Neoproterozoic arc magmatism on the northwestern margin of the Cathaysia block, represented by the Miaohou and Shanhou complexes.

Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China

Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg#=80–89) and the host dyke (Mg#=39, corresponding to Mg#=73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (0.26–1.09wt.%), Al2O3 (1.15–3.10wt.%) and Na2O (0.16–0.37wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenocrysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The 87Sr/86Sri (0.7035–0.7037) and εNdi (4.5–4.8) of the clinopyroxene macrocrysts with high Mg# (80–89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the wehrlites (Mg#=75–84, 87Sr/86Sri=0.7038–0.7041, εNdi=1.0–1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts.

Geochronology and geochemistry of Cretaceous Nanshanping alkaline rocks from the Zijinshan district in Fujian Province, South China: Implications for crust–mantle interaction and lithospheric extension

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for Nanshanping alkaline rocks from the Zijingshan district in southwestern Fujian Province (the Interior or Western Cathaysia Block) of South China. The Nanshanping alkaline rocks, which consist of porphyritic quartz monzonite, porphyritic syenite, and syenite, revealed a Late Cretaceous age of 100–93Ma. All of the rocks show high SiO2, K2O+Na2O, and LREE but low CaO, Fe2O3T, MgO, and HFSE (Nb, Ta, P, and Ti) concentrations. These rocks also exhibit uniform initial 87Sr/86Sr ratios of 0.7078 to 0.7087 and εNd(t) values of −4.1 to −7.2, thus falling within the compositional field of Cretaceous basalts and mafic dikes occurring in the Cathaysia Block. Additionally, these rocks display initial Pb isotopic compositions with a 206Pb/204Pbi ratio of 18.25 to 18.45, a 207Pb/204Pbi ratio of 15.63 to 15.67, and a 208Pb/204Pbi ratio of 38.45 to 38.88. Combined with the zircon Hf isotopic compositions (εHf(t)=−11.7 to −3.2), which are different from those of the basement rocks, we suggest that Nanshanping alkaline rocks were primarily derived from a subduction-related enriched mantle source. High Rb/Sr (0.29–0.65) and Zr/Hf (37.5–49.2) but relatively low Ba/Rb (4.4–8.1) ratios suggest that the parental magmas of these rocks were most likely formed via partial melting of a phlogopite-bearing mantle source with carbonate metasomatism. The relatively high SiO2 (62.35–70.79wt.%) and low Nb/Ta (10.0–15.3) ratios, positive correlation between SiO2 and (87Sr/86Sr)I, and negative correlation between SiO2 and εNd(t) of these rocks suggest that the crustal materials were also involved in formation of the Nanshanping alkaline rocks. Combined with geochemical and isotopic features, we infer magmatic processes similar to AFC (assimilation and fractional crystallization) involving early fractionation of clinopyroxene and olivine and subsequent fractionation of biotite-dominated assemblages coupled with a lesser amount of crustal contamination, thereby forming the Nanshanping alkaline rocks. The Nanshanping alkaline rocks appear to be associated with an extensional environment in the Cathaysia Block. This extensional regime could have resulted in the slab break-off and rollback of the subducting paleo-Pacific plate and the upwelling of the asthenospheric mantle, which induced partial melting of the enriched lithospheric mantle in the Cretaceous.

The geochemical and Sr–Nd isotopic characteristics of Eocene to Miocene NW Anatolian granitoids: Implications for magma evolution in a post-collisional setting

Early Eocene to Early Miocene magmatic activity in northwestern Anatolia led to the emplacement of a number of granitoid plutons with convergent margin geochemical signatures. Granitoid plutons in the area are mainly distributed within and north of the suture zone formed after the collision of the Anatolide-Tauride platform with the Pontide belt. We present geochemical characteristics of three intrusive bodies in the region in order to identify their source characteristics and geodynamic significance. Among these, the Çataldağ and Ilıca-Şamlı plutons are located to the north and the Orhaneli pluton is located to the south of the IAESZ (Izmir-Ankara-Erzincan Suture Zone). The plutons are calc-alkaline, metaluminous, and I-type with compositions from granite to monzonite. They display clear enrichments in LILE and LREE and depletions in HFSE relative to N-MORB compositions and have high 87Sr/86Sr and low 143Nd/144Nd ratios.The results of theoretical Fractional Crystallization (FC) model show that the samples are affected by fractionation of K-feldspar, plagioclase, biotite and amphibole. Assimilation and Fractional Crystallization (AFC) modeling indicates that the r value, the proportion of variable contamination to fraction, is high, indicating significant crustal contamination in the genesis of granitoid magmas. Combined evaluation of isotopic and trace element data indicates that the granitoids are the products of mantle-derived mafic magmas variably differentiated by simultaneous crustal contamination and fractional crystallization in lower to middle crustal magma chambers in a post-collisional setting.

Combined gravity and magnetic modeling over Pavagadh and Phenaimata igneous complexes, Gujarat, India: Inference on emplacement history of Deccan volcanism

A large number of igneous intrusions related to the Deccan magmatism are exposed in the western and central part of the Indian shield. Gravity and magnetic (G–M) surveys over some of these igneous intrusive bodies depict gravity high and bipolar magnetic anomalies as the most characteristic signatures. The present G–M survey was carried out over the Pavagadh and Phenaimata igneous intrusives. Associated with the Phenaimata complex, Bouguer gravity anomaly shows an elliptical shaped relative gravity high of about 40mGal and bipolar magnetic anomaly varies from South to North between −800nT and 1200nT. The joint G–M modeling reveals the presence of a dense mafic body (2.86g/cm3). This body is characterized by a remanant magnetization; the related inclination (I)=∼44° and declination (D)=∼160° may correspond with the 29R polarity chron of Deccan magnetostratigraphy. Remanant magnetization together with age data suggest that the Phenaimata igneous intrusive emplaced during the end of the main magmatism phase of Deccan. Over the Pavagadh, a circular gravity and magnetic lows of about −15mGal and −500nT respectively is reported for the first time which is surrounded by a gravity and magnetic high of about 30mGal and 350nT, respectively. The joint G–M modeling over the Pavagadh intrusive reveals the presence of a deep-seated cone shaped high-density (⩾3.0g/cm3) gabbroic body which might extend up to a great depth. Its top surface reaches up to a depth of about 10.0km. Overlying this body is a low-density (2.40g/cm3) rhyolite, which extends up to the surface and is the source for low gravity anomaly. It is surrounded by another high-density (2.89 and 3.02g/cm3) mafic bodies with reverse remanant magnetization direction (I=∼38° and D=∼152°). The modeled direction of remanant magnetization for the rhyolite (I=−32° and D=336°) and deeper gabbroic (I=−32° and D=340°) bodies show normal polarity. Measured magnetization direction for the mafic body surrounding the rhyolite relates to the middle reverse polarity (29R) chron. Inferred declination and inclination may then correspond to upper normal (29N), middle reverse (29R) and lower normal (30N) polarity chrons. Therefore, the magma forming the Pavagadh igneous complex was emplaced covering the major span of Deccan eruption. G–M model suggests that the magma chambers developed within the higher crustal levels and rhyolite originated from the underlying mafic magma through assimilation and fractional crystallization (AFC).

Heterogeneous magnesium isotopic composition of the lower continental crust: A xenolith perspective

We report 26 high‐precision whole‐rock Mg isotopic analyses for two suites of well‐characterized granulite xenoliths from Chudleigh and McBride, North Queensland, Australia, in order to constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the lower continental crust. Previous studies suggest that the Chudleigh granulites are a suite of cogenetic cumulates crystallized from mafic magmas that intruded into and assimilated the preexisting lower crust via combined assimilation and fractional crystallization (AFC). The δ26Mg values of the xenoliths range from −0.31 to −0.21‰ and correlate with radiogenic isotopes, reflecting mixing of mantle‐derived mafic magma (δ26Mg = −0.31‰) with preexisting isotopically heavy crustal materials (δ26Mg = ∼ +0.5‰) through the AFC process. The McBride granulites range compositionally from mafic to felsic, and originated as cumulates, solidified mafic/felsic melts, and restites that formed during basaltic underplating and reworking of preexisting lower crust. Their δ26Mg values vary widely from −0.72 to +0.19‰. The large Mg isotopic variation in the McBride xenoliths reflects both distinct source compositions and metamorphic enrichment of garnet, which is isotopically light. Based on these results, the lower continental crust has a heterogeneous Mg isotopic composition, with a weighted average δ26Mg of −0.18‰. The bulk continental crust, based on available data, has an average Mg isotopic composition of −0.19‰, and is slightly heavier than the mantle. The highly heterogeneous Mg isotopic distribution in the crust indicates that chemical weathering not only modifies the upper crust compositions but also significantly influences lower crust compositions through emplacement of upper crustal materials into the deep crust.

Early Permian post-collisional high-K granitoids from Liuyuan area in southern Beishan orogen, NW China: Petrogenesis and tectonic implications

Early Permian magmatism is one of the most important tectonothermal events in the Central Asian Orogenic Belt (CAOB). Early Permian granitic magmatism has been recognized from southern Beishan, NW China and they were emplaced between 275Ma and 279Ma according to LA-ICPMS zircon U–Pb dating. They are mainly metaluminous I-type and belong to the high-K calc-alkaline and shoshonitic series. The granitoids have calc-alkalic and alkali-calcic features. Their Sri values range from 0.7028 to 0.7047, εNd(t) values from −2.5 to +1.2 with Nd model ages (TDM) of 1.06–1.25Ga, suggesting a mixed magma source of juvenile material with old continental crust. Furthermore, some granitoids have weak heterogeneous zircon εHf(t) values (−1.7 to +9.6) and Hf model ages (TDM2=0.84–1.57Ga) that are also indicative of juvenile components with a small involvement of old continental crust. Based on geochemical and isotopic features, these high-K granitoids were derived from melting of heterogeneous crustal sources or through mixing of old continental crust with juvenile components and minor AFC (assimilation and fractional crystallization). The juvenile components probably originated from underplated basaltic magmas in response to asthenospheric upwelling. These Early Permian high-K calc-alkaline granitoids in the southern Beishan orogen were probably emplaced in a post-collisional extensional setting and suggest vertical continental crustal growth in the southern CAOB, which is the same or similar to most granitoids in CAOB. This study provides new evidence for determining the post-accretionary evolution of the southern CAOB.

Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

Normal mid-ocean ridge basalts (MORBs) from the Southwest Indian Ridge (SWIR) (48°E–64°E) have larger ranges of 187Os/188Os ratios from 0.1295 to 0.3023 than previously reported N-MORBs. 187Os/188Os ratios show positive correlations with 1/Os, suggesting possible contamination from a component with higher 187Os/188Os ratios. Mixing modeling between an unradiogenic MORB magma and seawater cannot reproduce the observed Os–Sr isotopic variations, suggesting that seawater alteration is not responsible for such radiogenic Os isotopic compositions. One sample plots between the isochrons of 400ka and 300ka, showing a possible radiogenic origin of 187Os after magma eruption. A group of MORBs have 187Os/188Os ratios negatively correlated with MgO. Such correlations can be modeled with starting 187Os/188Os ratio of 0.1357, bulk DOs of 15.3 and assimilation degree from 3% to 11% with a crystallization to assimilation ratio of 0.8. Os isotopic compositions of some MORBs from the SWIR reflect a significant influence of assimilation and fractional crystallization processes, mostly obscured the Os isotopic characteristics of the mantle sources. N-MORBs from the ultra-slow spreading SWIR and slow-spreading Mid-Atlantic Ridge show larger ranges of 187Os/188Os ratios than those from fast-spreading East Pacific Rise, reflecting different structures and chemical compositions of oceanic crusts due to different spreading rates of the ridges.

Late Cretaceous crustal growth in the Gangdese area, southern Tibet: Petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro

Recent studies of Gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern Tibet during the Jurassic–Early Eocene. However, the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes. Here, we report on the Zhengga diorite–gabbro suite from the Gangdese area. They consist of plagioclase (labradorite and anorthite), amphibole, clinopyroxene, biotite and minor magnetite, epidote, zircon and apatite. LA-ICP-MS zircon U–Pb dating for two samples gives a ca. 94Ma age for the Zhengga intrusive rocks, i.e., the Late Cretaceous. Apart from one high-SiO2 (52.2wt.%) diorite sample with slightly high K2O (1.72wt.%) and initial 87Sr/86Sr (0.7068) and low εNd(t) (−5.6) values, the gabbro samples are geochemically characterized by low SiO2 (39.8–50.1wt.%) and K2O (0.3–1.1wt.%), strongly negative Nb–Ta and positive Sr anomalies, and uniform initial 87Sr/86Sr (0.7043–0.7048). The gabbros can be divided into two groups: Group I gabbros with relatively low total rare earth element (REE), Rb and SiO2 contents and positive Eu anomalies, and Group II gabbros with slightly higher total REE, Rb and SiO2 contents and negative Eu anomalies. The Group I gabbros have εNd(t) (+1.7 to +4.1), and zircon εHf(t) (+6.5 to +11.1) and δ18O (5.89 to 7.24‰) values, which are slightly different to those of the Group II gabbros (−0.2 to +2.0, +2.9 to +6.5 and 6.24 to 7.05‰). Trace element compositions of amphibole and clinopyroxene grains suggest that the Zhengga mafic magmas contained a significant fluid-transported component, probably released from subducted oceanic lithosphere. We suggest that the parental magmas of the Zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts/fluids. The Group I gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas, with insignificant crustal contamination, whereas the Group II gabbros were probably produced by assimilation and fractional crystallization (AFC) processes from mafic magmas that were geochemically similar to the Group I gabbros. Pre-collisional underplating of mantle-derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late Late Cretaceous–Cenozoic granitoids of the Gangdese batholiths. This study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust.

Silicon isotopes in granulite xenoliths: Insights into isotopic fractionation during igneous processes and the composition of the deep continental crust

The silicon (Si) cycle is of great current interest but the isotopic composition of the continental crust has not been determined. Magmatic differentiation generates liquids with heavier Si and the lower crust, thought to be dominated by cumulates and restites, is predicted to have a light isotopic composition. This is borne out by the composition of many types of granite, which appear to have relative light Si for their silica content. Here we report the Si isotopic compositions of two granulite facies xenolith suites, from the Chudleigh and McBride volcanic provinces, Australia, providing new constraints on deep crustal processes and the average composition of the deep continental crust.The xenoliths display a range of isotopic compositions (δ30Si=−0.43‰ to −0.15‰) comparable to that measured previously for igneous rocks. The isotopic compositions of the McBride xenoliths reflect assimilation and fractional crystallisation (AFC) and/or partial melting processes. Silicon and O isotopes are correlated in the McBride suite and can be explained by AFC of various evolved parent melts. In contrast, the Chudleigh xenoliths have Si isotope compositions predominantly controlled by the specific mineralogy of individual cumulates. Using the xenolith data and a number of weighting methods, the Si isotope compositions of the lower and middle crust are calculated to be δ30Si=−0.29±0.04‰ (95% s.e.) and −0.23±0.04‰ (95% s.e.) respectively. These values are almost identical to the composition of the Bulk Silicate Earth, implying minimal isotope fractionation associated with continent formation and no light lower crustal reservoir.

PETROMODELER (Petrological Modeler): a Microsoft® Excel© spreadsheet program for modelling melting, mixing, crystallization and assimilation processes in magmatic systems

PETROMODELER (Petrologic Modeler) is a Microsoft® Excel© spreadsheet program which numerically and graphically models magmatic processes including melting, crystallization, assimilation and mixing by using trace elements and isotopic ratios. Melting models include (a) batch, (b) dynamic (continuous) and (c) fractional melting for (1) instantaneous and (2) cumulate melts, (3) residual solid and (4) total residue. These models can also be used to treat modal and non-modal melting. Crystallization processes modelled in the program include: (1) perfect equilibrium and (2) perfect fractional crystallization (PEC and PFC), (3) equilibrium crystallization-imperfect fractional crystallization (EC-IFC), (4) zoned crystallization-imperfect fractional crystallization (ZC-IFC), and (5) combined assimilation and fractional crystallization (AFC). Mixing between two end-member compositions can also be modelled by the program. The main advantages of the program are that; (1) crystallization and mixing processes can be performed on a starting composition which may be chosen from; (a) any melting model result, or (b) any sample composition entered into the 'samples' tables, (2) the results of any model can be exported as a graphic file (GIF) and as tables, (3) changes in any parameters are simultaneously updated onto all diagrams and tables. PETROMODELER also calculates other useful parameters, such as normative mineralogy, Mg#, Eu/Eu*, \epsilonSr and \epsilonNd, \sigma(DM) (depleted mantle Nd model ages) of a given sample. Some classification diagrams for volcanic rocks are also included in the program. Conversion of element abundances on the basis of wt% and ppm can also be performed.

Evolution and origin of the Miocene intraplate basalts on the Aleppo Plateau, NW Syria

The source of intraplate basalts has long been a controversial topic, particularly in continental settings where ambiguity increases because both crustal contamination and crystal fractionation may mask important source characteristics. We present geochemical data to constrain the source and the chemical evolution of the continental intraplate magmas from the Aleppo Plateau and vicinity, NW Syria. New 40Ar/39Ar ages, coupled with published 40Ar/39Ar and K–Ar ages, reveal two discrete Miocene volcanic phases, ~19–18Ma (Phase 1) and ~13.5–12Ma (Phase 2), in the studied area. New chemical and isotopic compositions [87Sr/86Sr=0.7036–0.7051, εNd=+4.5 to +1.1 and (187Os/188Os)t=0.151–0.453] of the lavas reflect the unequivocal influence of crustal assimilation and fractional crystallisation (AFC). Despite the effects of the AFC processes, there still appear to be some differences between the most-primitive, least contaminated magmas of the two volcanic phases, interpreted as a result of source heterogeneity. Whereas the Phase 1 lavas, with relatively high Si, low Ti and trace-element contents, are consistent with partial melting of a largely peridotitic mantle source, the origin of the Phase 2 lavas is more complicated. The latter are characterised by a source component depleted in Si and enriched in Ti, Fe, Ca, P, alkalis, light and middle rare earth elements (REEs) relative to heavy REEs and with sub-chondritic Th–(U)/Nb, Pb/Ce and Zr/Sm. They approach compositions of experimental melts of amphibole-rich metasomatic veins. The compositional variations among the most primitive Phase 2 lavas are difficult to reconcile with varying degrees of partial melting of either the metasomatic veins or peridotite, but could be explained if partial melts of both lithologies were variably mixed, a scenario that could be sensibly envisioned as ascending (peridotitic) plume/asthenosphere derived melts assimilating highly fusible metasomatic veins during their traverse through the lithosphere. This process can be loosely quantified by trace-element forward partial melting modelling that suggests mixing of up to 80% metasomatic melts derived from ~40% melting of amphibole-rich metasomatic veins (which themselves were inevitably compositionally and mineralogically heterogeneous) with 20% plume/asthenospheric melts derived from ~7% melting of a garnet peridotite. Within the compressional framework of northern Arabia, invocation of diapiric material reasonably accounts for the generation of the intraplate basalts in Syria. Derivation of the Phase 2 hybrid melts was probably triggered by lateral flow of this diapiric material beneath the lithosphere subsequent to its arrival, with the migrating flow-front controlling the locus of volcanism. The increase in degree of Si-undersaturation with time for the Phase 1 and Phase 2 lavas is best explained by decreasing temperatures of this flow-front that resulted in less melt contribution from the diapiric mantle while the amphibole-rich veins within the lithosphere continued to be easily fusible, although we cannot totally exclude the possibility that the Phase 2 volcanism tapped a vein-richer domain which formed subsequent to the Phase 1 volcanism.

Intraplate crustal remelting as the genesis of Jurassic high-K granites in the coastal region of the Guangdong Province, SE China

The origin and tectonic significance of high-K granites (>3wt% K2O at 70wt% SiO2), calc-alkaline I-type granites in particular, remain controversial. This paper takes granitic plutons distributed in the coastal region of the Guangdong Province of southeastern China as examples to explore the genesis of such rocks. SIMS zircon U–Pb geochronological data show that the granites were emplaced at 166–159Ma. These granites can be subdivided generally into two groups on the basis of integrated mineralogical, geochemical, whole-rock Sr–Nd isotopic and in situ zircon Hf–O isotopic studies. The group A granites (SiO2=64–72wt%) are characterised by their common occurrence of amphibole (±titanite) and dominantly metaluminous feature (A/CNK=0.85–1.03). They are high in K2O (3.5–7.0wt%) and K2O/Na2O (>1), and have trace element concentrations (e.g., Nb, Y, Zr and Ga) similar to typical I-type granites in the Lachlan Fold Belt, southeastern Australia. Their whole-rock ISr (0.7057–0.7077) and εNd(t) (−6.46 to −3.13) are less evolved than many coeval granites in this region. As in situ zircon Hf–O isotopes show little evidence of magma mixing, these granites with low zircon δ18O (6.3−7.9‰) and high εHf(t) (−5.9 to −0.2) could have been generated from melting of oxidised high-K basaltic rocks. The group B granites, emplaced to the east of group A granites, are dominantly weakly peraluminous (A/CNK=1.00–1.05). They have higher SiO2 (70–76wt%), less common or absence of amphibole, higher zircon δ18O (6.6−9.0‰) and lower εHf(t) (−11.4 to −5.9) than the group A granites. Zircon Hf–O isotope data reveal that the group B granites contain higher percentage of supracrustal materials than those of the group A, but the variations of major and trace elements do not support an assimilation and fractional crystallization (AFC) model. Instead, the group B granites, with features transitional between typical I-type and S-type granites, were most likely formed in a region where there was physical juxtaposition between infracrustal metaluminous and supracrustal peraluminous source rocks. Thus, granites of both groups represent products of crustal reworking likely due to asthenosphere upwelling and/or underplating and intrusion of mafic magmas. The close association in time and space of these granites with OIB-like basaltic rocks and the secular compositional change of Jurassic basaltic rocks in the region suggest that these rocks probably formed in an intraplate extensional setting resulted from the delamination of a flat-subducted oceanic slab.

A mathematical model of trace element and isotopic behavior during simultaneous assimilation and imperfect fractional crystallization

The process of coupled assimilation and fractional crystallization (AFC) is one of the most popular petrogenetic concepts that explains magmatic differentiation. Conventional geochemical models for this process assume that crystals are removed instantaneously from the magma body as they are produced; however, recent advances in isotopic microanalysis have clarified that the crystals are suspended within the magma body for a certain period, affecting the whole-rock composition in response to the intra-grain isotopic zoning. This study develops a mass balance model for simultaneous assimilation and imperfect fractional crystallization (AIFC) to describe the effects of suspended crystals on the path of magma evolution. The mass balance differential equations for the liquid and suspended crystals are solved simultaneously. The analytical solution of the AIFC equations gives a quantitative account of the evolution paths of trace elements and isotopes within bulk crystals, liquid, and magma (crystals plus liquid). The chemical path of the magma differs markedly from that predicted by the conventional AFC model.

Discovery of an adakite-like pluton near Dongqiyishan (Beishan, NW China) — Its age and tectonic significance

The Dongqiyishan granitic pluton is exposed in the Eastern Beishan area, in the central part of the Southern Central Asia Orogenic Belt. SIMS U–Pb zircon dating indicates that the Dongqiyishan pluton was emplaced at 356±2Ma. Geochemically the pluton is calc-alkaline and characterized by high SiO2 (>65%), Al2O3 (mostly >15%), Na2O/K2O ratios (1.3–3.9) and Sr (generally >400ppm). Samples display positive to weakly negative Eu anomalies (δEu=0.81–2.32), low HREE, Y and Yb (Y<18ppm, Yb<1.9ppm), are enriched in LREE, LILE (Rb, Ba, Sr, U, and K), Zr, and Hf, and depleted in HFSE. They have relatively low εNd(t) values (–2.9–+0.6) and relatively high (87Sr/86Sr)i ratios (0.70581–0.70749). Together, these geochemical traits suggest an adakite-like composition that reflects melting of subducted oceanic crust and the involvement of older continental material. Assimilation and fractional crystallization (AFC) modeling requires significant crustal assimilation to modify an initial MORB-derived andesitic melt to produce the hybrid granitic melt, e.g. — the Dongqiyishan pluton. A back-arc basin to the south (represented by the Yueyashan–Xichangjing ophiolitic belt) closed before the Early Carboniferous Dongqiyishan adakite-like pluton was emplaced into the Silurian volcanic arc. Closure of the back-arc basin potentially provides the continental component seen in the isotopic signature of the Dongqiyishan pluton and required by the AFC modeling. Post-Ordovician flattening of the subducting slab together with crustal thickening from closure of the back-arc basin, led to the formation of a tightly coupled subducting slab and lower crust which resulted in the formation of Dongqiyishan adakite-like pluton. The Dongqiyishan pluton is consistent with southward subduction (represented by the Xiaohuangshan ophiolitic belt) and indicates that oceanic crust was still being produced in the Paleo-Asian Ocean during the Early Carboniferous. Adakite-like granitic rocks may be more abundant than previously thought in the Beishan region.

Early Jurassic high-K calc-alkaline and shoshonitic rocks from the Tongshi intrusive complex, eastern North China Craton: Implication for crust–mantle interaction and post-collisional magmatism

The Tongshi intrusive complex, located within the western Shandong Province (Luxi Block) in the eastern North China Craton, comprises high-K calc-alkaline series (fine-grained quartz monzonite and porphyritic quartz monzonite) and shoshonitic series (coarse- to fine-grained porphyritic syenites). Here we report comprehensive data on petrology, geochemistry, Sr–Nd–Pb isotopes and zircon U–Pb and Hf isotopic compositions from the intrusive complex. LA–ICPMS zircon U–Pb ages show that this complex was emplaced at 180.1–184.7Ma. The fine-grained quartz monzonite and porphyritic quartz monzonite have similar major and trace elements features, implying a similar petrogenetic history. Coupled with the widespread Neoarchean inherited zircons in these rocks, the high SiO2 and Na2O as well as the low MgO contents and low Pb isotopic ratios ((206Pb/204Pb)i=15.850–16.881, (207Pb/204Pb)i=14.932–15.261, (208Pb/204Pb)i=35.564–36.562) of the quartz monzonites suggest an origin from ancient tonalite–trondhjemite–granodiorite (TTG) crust. However, their higher Nd and Hf isotopic ratios (εNd (t)=−11.7 to −7.0, εHf (t)=−25.0 to −10.3) as compared to the basement rocks indicate input of enriched lithospheric mantle-derived materials. The coarse- to fine-grained porphyritic syenites were derived from similar sources as inferred from their comparable major and trace elements contents as well as the Nd, Hf and Pb isotopic compositions. The Neoarchean inherited zircons and depletion of Nb, Ta, P and Ti in these rocks indicate the involvement of ancient crust. However, the high Nd and Hf isotopic ratios (εNd (t)=−0.8 to 1.5, εHf (t)=−4.4 to 4.8) coupled with high Pb isotopic compositions ((206Pb/204Pb)i=18.082–19.560, (207Pb/204Pb)i=15.510–15.730, (208Pb/204Pb)i=37.748–39.498) suggest that the porphyritic syenites were mainly derived from an asthenospheric mantle. Based on the geochemical and isotopic features, a magmatic process similar to MASH (melting, assimilation, storage and homogenization) is proposed for the fine-grained quartz monzonite and AFC (assimilation and fractional crystallization) is envisaged for the porphyritic quartz monzonite and porphyritic syenites. Our data suggest the interaction of various crustal and mantle sources including lower to upper crust, lithospheric mantle and asthenospheric mantle in the generation of the Tongshi intrusive complex. Sharing common characteristics with other post-collisional magmatic rocks, the high-K calc-alkaline and shoshonitic rocks from the Tongshi complex are considered to have originated in post-collisional setting, marking the beginning of lithospheric destruction in the inner domains of the NCC during Mesozoic.

Petrogenesis of gabbro and orthopyroxene gabbro from the Phenai Mata Igneous Complex, Deccan volcanic province: Products of concurrent assimilation and fractional crystallization

We report the occurrence of orthopyroxene gabbro from the Phenai Mata Igneous Complex (containing thoeliitic and alkaline rocks) that occur within Deccan Traps. The P-T calculations based on two pyroxene thermometry vary from 8.5±1.0 kbar and 963±39 °C. These gabbroic rocks exhibit high Mg# (0.67 to 0.71). But their primary magma signature can be negated due to their high SiO2 (> 50 wt %), low Ni (32–35 ppm) and Cr (105–182 ppm) contents. Further, simple fractional crystallization was not responsible for the modification of the magma. Modeling carried out using trace element concentrations revealed that concurrent assimilation and fractional crystallization (AFC) was responsible for the genesis of these rocks. Small pods of magma could have accumulated in the crustal portions and concurrent assimilation and fractional crystallization have taken place in the generation of gabbro and orthopyroxene gabbro in the present study area.

Geochemical and Sr–Nd isotopic constraints on the genesis of the Cenozoic Linzizong volcanic successions, southern Tibet

The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet have been conventionally regarded as the products of northward subduction of the Neotethyan oceanic slab beneath South Asia. This study reports geochemical data of 100+ volcanic rocks from the Lhasa terrane to better constrain the temporal–spatial distribution and petrogenesis of the Linzizong volcanism. The Linzizong volcanic successions consist dominantly of calc-alkaline rocks that erupted from ca. 69 to 43Ma and show typical arc-lava geochemical features marked with LILE enrichment and HFSE depletion in the spidergram. Their Sr and Nd isotope ratios [εNd(T)=+3.3 to −2.4; ISr=0.7048–0.7072] are generally similar to those of the associated Gangdese I-type granitoids. The Linzizong volcanism is characterized by a flare-up period (ca. 50Ma) that shows significant geochemical variations, manifested by the coexistence of five types of volcanic rocks: (1) the main suite of calc-alkaline rocks [SiO2=45–80wt.%; La=12–45ppm; εNd(T)=+3.8 to −4.9; ISr=0.7037–0.7105] that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization (AFC) processes with >10% crustal contamination and/or magma mixing; (2) the low-K, low-REE suite (SiO2=48–61wt.%; K2O=0.5–1.1wt.%; La=7–10ppm) that has the highest Nd isotope ratios [εNd(T)=+5.9 to 3.5], suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere; (3) the shoshonitic suite [SiO2=53–71wt.%; K2O=3.8–6.7wt.%; εNd(T)=−2.8 to −6.1] from small-degree melting of the metasomatized lithospheric mantle; (4) the high-REE suite [SiO2=61–75wt.%; La=36–87ppm; εNd(T)=−2.6 to −3.2] originating from remelting of a newly underplated basaltic lower crust; and (5) the evolved suite of rhyolitic flows and ignimbrites (SiO2>65wt.%) that have the lowest εNd(T) values of −14 to −18, representing remelting products of the basement or continental crust of the Lhasa terrane. Such geochemical heterogeneities are attributed to breakoff of the subducted Neotethyan slab under southern Tibet that occurred in the early stage of the India–Asia collision.

Investigation into the petrogenesis of Apollo 14 high-Al basaltic melts through crystal stratigraphy of plagioclase

The petrogenesis of Apollo 14 high-Al basaltic melts was studied using crystal stratigraphy, which involves textural (crystal size distributions — CSDs) and chemical analyses (electron microprobe and laser ablation inductively coupled plasma mass spectrometry). The samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321, and impact-generated crystalline samples 14073, 14276 and 14310. Plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories. Plagioclase crystals were analyzed (core-to-rim compositions where possible) to test and refine petrogenetic models based upon whole-rock compositions (Groups A, B, and C designations) and to investigate basalt 14072 and impact-melt crystallization. Textural studies have shown that each basalt group has distinctive plagioclase CSDs, which are in turn distinctive from those of the impact melts. Evolution of the individual basaltic melts was studied by comparing the equilibrium-melt compositions (calculated from plagioclase compositions using relevant partition coefficients) to fractional crystallization (FC) and assimilation and fractional crystallization (AFC) models. Petrogenetic modeling of trace elements in Group A basalts revealed that petrogenesis continued beyond 40% total crystallization required to model whole-rock compositions, and that there were open-system processes that affected the magma during plagioclase crystallization. Petrogenetic modeling of pristine high-Al basalts (14072 and Groups A, B and C) using trace elements shows that the equilibrium-melt compositions do not fall on a single AFC or FC trajectory. This is consistent with fluctuating degrees of assimilation (i.e., variable r-values) and/or variable assimilant compositions during petrogenesis. Petrogenetic modeling reveals that the impact melts experienced only closed-system fractional crystallization. This work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis.