Host Felsic Magma Research Articles

Field, textural, geochemical, and isotopic constraints on the origin and evolution of the magmatic microgranular enclaves from the Gharib Granitoid Complex, North Eastern Desert, Egypt

The Neoproterozoic Gharib Granitoid Complex (GGC), in the North Eastern Desert (NED) of Egypt, is characterized by the occurrence of varieties of granitoids. Monzogranite, the most abundant granite variety, contains abundant magmatic microgranular enclaves (MMEs) of mafic to intermediate composition. The MMEs are more mafic than their host and marked by a greater amount of amphibole and biotite. They are finer grained and have porphyritic/poikilitic textures.The sharp, crenulated, and fine-grained quenched contacts of the elongated, rounded, and ellipsoid form of the MMEs with their felsic host indicate rapid cooling during a magma mixing/mingling event. Micro-textural evidence supports the mechanical transfer of minerals that crystallized in felsic magma to the mafic melt (e.g., feldspar phenocrysts cross-cutting the MME-host boundaries, stubby apatite, quartz ocelli, and poikilitic outermost rims of K-feldspar phenocrysts), supercooling/quenching process (e.g., acicular apatite and elongated hornblende and biotite crystals), and disequilibrium growth (e.g., hornblende-biotite clots replaced early crystallized clinopyroxene), which also favour a magma mingling/mixing origin. Linear to curvilinear chemical variations of the MMEs and the host monzogranites on Harker diagrams, highly similar trace element contents, and Sr-Nd isotopic systematics together with mineral-chemical features also support mixing/mingling as the main process in magma genesis and strongly suggest that the MMEs were supercooled hybrid globules within cooler, partially crystallized host felsic magma. The crust-like geochemical signatures (i.e., SiO2 content, metaluminous nature, and lack of upper crustal xenoliths and typical peraluminous minerals), along with low [MMEs: (87Sr/86Sr)initial = 0.70136–0.70373; monzogranites: (87Sr/86Sr)initial = 0.70202–0.70320)] along with zircon and apatite U-Pb age data indicate the possible involvement of juvenile lower continental crust (LCC) of the Arabian-Nubian Shield (ANS) in their genesis. The medium- to high-K calc-alkaline affinity, the LILE enrichment, and the HFSE depletion along with low Nb/La ratios and positive εNd(t) (MMEs = +4.01 - +5.62, monzogranites = +2.29 - +6.03) indicate the contribution of depleted lithospheric mantle in the magma genesis, whereas the low δEu and fairly high δCe values, stable La/Sm, and variable Sr/Th ratios in apatites from the monzogranites and MMEs indicate the contribution of volatile/fluids from the remnants of the oceanic slab in facilitating melting in the source region. The genesis of the monzogranites and MMEs started with asthenosphere upwelling, followed by melting of the remnants of lithospheric mantle, and underplating of mantle magma, which led to partial melting of the LCC. Multi-element modelling postulates that the low degree mixing between partially crystallized mantle magma and LCC-derived felsic melt (Fmix ∼ 0.1) could produce the parent magma for the monzogranites. Further mixing/mingling of the crystal-charged monzogranite parent magma with new pulses of mafic melts (Fmix ∼ 0.25) developed a hybrid zone and formed the hybrid MMEs. Thus, the final episode of the ANS evolution was associated with a complex post-collisional interplay between asthenosphere, lithospheric mantle, and the LCC caused by the removal of delaminated lithospheric root and dense lower crust following the collision between East and West Gondwana in the Neoproterozoic time.

Magma hybridization and crust-mantle interaction revealed by mineralogical and geochemical footprints: a case study from the South Qilian Accretionary Belt at the northern Tibetan Plateau

ABSTRACT The relationship between mafic magmatic enclaves (MMEs) and host granitoids is ongoing debate due to their significant roles in understanding deep magmatisms and geodynamic processes. This paper reports on Early Palaeozoic granitoids and associated MMEs from the Delingha pluton in the South Qilian Accretionary Belt (NW China) to constrain their petrogenesis and draw implications. The MMEs, occurring as ellipsoidal to rounded shapes, are andesitic and have the same crystallization age (~445 Ma) and mineral assemblage as the host granitoids. However, minor isotopic differences between them and compositional and textural disequilibria on minerals (e.g. plagioclase and amphibole) do not support a cumulate origin that the MMEs were formed at earlier stage of the same magmatic system, but forcefully reflect magma hybridization processes between mafic and felsic magmas, during which the hybridized magma was broken up into discrete enclaves by convective motion in the host felsic magma. Accordingly, the primitive compositions of the MMEs likely originated from metasomatized mantle wedge source, while mantle-derived melt underplated and partially melted the lower continental crust to form the host rocks with a certain mantle contribution. Thus, the Delingha pluton is the product of crust-mantle interaction at an active continental margin subduction setting rather than syn-collisional environment, indicating that subduction of the South Qilian Ocean located between the Central Qilian Block and the Oulongbuluke Block may have lasted till to ~445 Ma.

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.

Redox series assessment, petrogenetic, and geodynamic appraisal of Neoarchean granites from the Bundelkhand Craton, Central India: Constraints from phase petrology and bulk rock geochemistry

Based on field relationships, microstructure, phase petrology, and whole‐rock elemental geochemistry, we have unravelled the redox conditions and petrogenetic evolution of Neoarchean granites (sensu lato) from the central part of the Bundelkhand Craton, Central India. The studied Neoarchean granites are sanukitoids (sensu lato), high‐K, low‐magnesian granites (HKLMG), and oxidized A‐type granites (OAG). Chemistry of biotite in conjunction with amphibole compositions from sanukitoids indicates their crystallization in calc‐alkaline felsic host magma formed in a subduction setting at elevated fugacity of oxygen (fO2), buffered at +2 fayalite‐magnetite‐quartz (FMQ), corresponding to magnetite series granite. Chemistry of biotite and amphiboles from HKLMG suggests that host melts of these phases buffered above +1 FMQ, that is, moderate fO2 condition corresponding to magnetite series granites. The nature of the HKLMG magma is peraluminous to alkaline which originated at a relatively high‐P condition. The biotites from OAG exhibit pronounced Al‐Fe substitution buffered above +0.5 FMQ to represent oxidized A‐type host granite magma, belonging to magnetite series granite. The sanukitoids bear high ferromagnesian minerals (FeOt + MgO + MnO + TiO2 (FMMT) >4.5; Mg# = 28–45), and enriched incompatible elements without significant Eu anomalies, probably originated from melt‐enriched mantle source in a subduction setting. The HKLMG are characterized by relatively low Mg#, TiO2, high K‐LILEs‐REEs with negative Eu anomalies, and high Th/Nb, Ce/Pb, and La/Nb ratios, which suggest mixed source signatures, derived from the enriched mantle and continental crust. A‐type granites (OAG) bear high NK/A (molar Na2O + K2O/Al2O3 ~ 1.17), FeOt/MgO, K2O/Na2O, Ga/Al, and Ce–Y, which indicate the emplacement of OAG magmas in syn‐ to post‐collisional tectonic environments. Thus, the Neoarchean granites from the Bundelkhand Craton represent a collage of subduction related sanukitoids to high‐K magnesian granites, and syn‐ to post‐collisional oxidized A‐type granites.

Schedule of Mafic to Hybrid Magma Injections Into Crystallizing Felsic Magma Chambers and Resultant Geometry of Enclaves in Granites: New Field and Petrographic Observations From Ladakh Batholith, Trans-Himalaya, India

Diorites, granites, and associated magmatic enclaves and dykes constitute the bulk of the Ladakh Batholith, which is an integral part of the Trans-Himalayan magmatic arc system. In this paper, geometry of microgranular enclaves hosted in the granites has been examined from the Leh-Sabu-Chang La and surrounding regions of the eastern Ladakh Batholith to infer the mechanism and schedule of mafic to hybrid magma injections into evolving felsic magma chambers and the resultant enclave geometry. Mafic and/or hybrid magmas injected into felsic magma at low volume fraction ( 0.65) of crystals. A large rheological difference between coeval felsic and mafic magmas inhibits much interaction. Mafic magma progressively crystallizes and evolves while minimizing thermal and rheological differences. Consequently, the felsic-mafic magma interaction process gradually becomes more efficient causing dispersion of enclave magma globules and undercooling into the partly crystalline felsic host magma. Thus, the evolution of the Ladakh Batholith should be viewed as multistage interactions of mafic to hybrid magmas coeval with felsic magma pulses in plutonic conditions from its initial to waning stages of evolution.

Early Cretaceous mantle upwelling and melting of juvenile lower crust in the Middle-Lower Yangtze River Metallogenic Belt: Example from Tongshankou Cu-(Mo[sbnd]W) ore deposit

The linkage between intracontinental extension and the Early Cretaceous Cu-Mo-W polymetallic metallogenesis in the Middle-Lower Yangtze River Belt (MLYRB) has long been a subject of controversy due to the lack of convincing petrogenetic evidence to identify the nature of magmatic sources and their geological histories during extensional mantle upwelling. Here we present new zircon UPb ages, isotopic and geochemical data for granodiorites, quartz diorites and mafic microgranular enclaves (MMEs) in the Tongshankou area. Comparing the MMEs with their host porphyries, the different ratios of incompatible elements and the similar formation ages, coupled with quenched margins and the xenocrysts in the MMEs, indicate that the MMEs was most likely formed by mixing between mafic magma and their host felsic magma. The MMEs share similar geochemical and isotopic characteristics with the Cretaceous mafic rocks from MLYRB, indicating that MMEs were mostly derived from an enriched lithospheric mantle source without adakitic characteristics. Mixing of a crustal melt derived by melting of an amphibolite bearing juvenile lower crust with a mantle melt derived from melting of enriched lithospheric mantle can account for the generation of the Tongshankou prophyries. The melting of juvenile mafic lower crust and enriched lithospheric mantle is suggested to be caused by upwelling of asthenospheric mantle and the reactivity of trans-lithospheric faults in the intracontinental extensional environment. Our results therefore highlight that juvenile mafic lower crust beneath the Yangtze plate is one of the likely source for ore-forming magmatic rocks in the Early Cretaceous.

Evidence of mingling between contrasting magmas in the Ribeirão do Óleo Pluton, Coastal Terrane and the tectonic implications on the Ribeira Belt, Brazil

Gneiss–migmatitic rocks and granites between the Cubatao and Itariri shear zones characterize the Mongagua Complex-Coastal Terrane. Three major units are defined: (1) the Itariri Suite (IS) composed of monzogranites, granodiorites, tonalites, and locally gneiss–migmatitic rocks; (2) the Areado pluton composed of monzogranite and locally tonalite; and (3) the Ribeirao do Oleo Pluton mainly composed of monzogranites, which is the subject of this article. Combined field observations, petrography, Sr–Nd–Hf isotopes, U–Pb zircon data, and preliminary geochemical studies are presented for the Ribeirao do Oleo Pluton (ROP), intrusive in granite-gneisses of the IS. It is a plutonic association of weakly peraluminous, medium-to-high-K biotite monzogranites that surround microgranular intermediate enclaves, which are products of magma mingling. Different from the most examples of magma mingling described in the literature, the ROP mingling did not result from the intrusion of hot mafic magma into colder host felsic magma. Conversely, the emplacement of the monzogranite started most likely during the medium-to-final stage of crystallization of the quartz microdiorite. A detailed study of LA-ICP-MS U–Pb zircon ages, including zircon morphology and internal structure, was useful to determine more precisely the emplacement sequence and the relation between the units of the ROP. Intermediate and acid facies have Paleoproterozoic and Neoproterozoic inheritances, moreover a mixture of xenocrysts, antecrysts, and autocrysts, indicative of a complex evolution of the ROP. Several magma crystallization pulses are recorded at 655–624 Ma, 627–601 Ma, and 613–586 Ma. The negative eHf(t) ranging from − 17.26 to − 12.51 in monzogranites, eNd(t) values of − 10.66 to − 7.83 at t = 600 Ma in both facies, 87Sr/86Sr of ca. 0.708 for the monzogranite and 0.706 for the quartz microdiorite indicate a strong crustal contribution in the genesis of intermediate and acid magmas of this pluton.

Redox Condition, Nature and Tectono-magmatic Environment of Granitoids and Granite gneisses from the Karbi Anglong Hills, Northeast India: Constraints from Magnetic Susceptibility and Biotite Geochemistry

ABSTRACT The Karbi Anglong hills (erstwhile Mikir hills) in northeast India are detached and separated from the Meghalaya plateau by a NW-SE trending Kopili rift. The Karbi Anglong hills granitoids (KAHG) and its granite gneissic variants belong to Cambrian plutons formed during Pan-African orogenic cycle, which commonly intrude the basement granite gneisses and Shillong Group metasediments. The KAHG can be broadly classified into three major granitoid facies viz., coarse grained porphyritic granitoid, medium grained massive non-porphyritic granitoid, and granite gneiss, which share a common mineral assemblage of plagioclase-K-feldspar-quartz-biotite±hornblende-apatite-titanite-zircon-magnetite but differ greatly in mineral proportion and texture. Modal mineralogy of KAHG, granite gneiss and basement granite gneiss largely represents monzogranite and syenogranite. The magnetic susceptibility (MS) of the KAHG, granite gneiss and basement granite gneiss varies widely between 0.11×10−3 and 43.144×10−3 SI units, corresponding to ilmenite series (<3×10−3 SI; reduced type) and magnetite series (>3×10−3 SI; oxidized type) of granitoids respectively. The observed MS variations are most likely intrinsic to heterogeneous source regions, modal variations of orthomagnetic and ferromagnetic minerals, and tectonothermal and deformational processes that acted upon these rocks. The primary and re-equilibrated compositions of biotites from the KAHG, granite gneiss and basement granite gneiss suggest calc-alkaline, metaluminous (I-type) nature of felsic host magma formed in a subduction or post-collisional to peraluminous (S-type) host magma originated in syn-collisional tectonic settings, which were evolved and stabilized between FMQ and NNO buffers typically corresponding to reducing and oxidising magma environments respectively.

Geochemical and isotopic constraints on the role of juvenile crust and magma mixing in the UDMA magmatism, Iran: evidence from mafic microgranular enclaves and cogenetic granitoids in the Zafarghand igneous complex

The Zafarghand Igneous Complex is composed of granite, granodiorite, diorite, and gabbro that contain many mafic microgranular enclaves. This complex was emplaced during the late Oligocene (24.6 Ma) to form part of the Urumieh–Dokhtar magmatic arc of Central Iran. The enclaves have spheroidal to elongated/lenticular shapes and are quenched mafic melts in felsic host magma as evidenced by fine-grained sinuous margins and (or) locally transitional and diffuse contacts with the host rocks, as well as having disequilibrium textures. These textures including oscillatory zoning with resorption surfaces on plagioclase, feldspar megacrysts with poikilitic and anti-rapakivi textures, mafic clots, acicular apatites, and small lath-shaped plagioclase in larger plagioclase crystals all indicate that the enclaves crystallized from mafic magma that was injected into and mixing/mingling with the host felsic magma. The studied rocks have calc-alkaline, metaluminous compositions, with an arc affinity. They are enriched in large ion lithophile elements, light rare-earth elements, and depleted in high field strength elements with significant negative Eu anomalies. The Sr–Nd isotopic data for all of the samples are similar and display ISr = 0.705123–0.705950 and eNd (24.6 Ma) = − 1.04–1.03 with TDM ~ 0.9–1.1 Ga. The host granites and enclaves are of mixed/mingled origin and most probably formed by the interaction between the juvenile lower crust with a basaltic composition and old lower or middle continental crust as a major component and lithospheric mantle as a minor component; this was followed by fractional crystallization and possibly minor crustal assimilation. The source seems to be comprised of about 90–80% of the basaltic magma and about 10–20% of lower/middle-crust-derived magma. Geochemical characteristics indicate that the intrusion of these rocks from a subduction zone setting below the Central Iran micro-continent was related to an active continental margin, although was transitional to a transtensional setting possibly due oblique convergence to slab rollback or break-off.

Origin of the Eocene porphyries and mafic microgranular enclaves from the Beiya porphyry Au polymetallic deposit, western Yunnan, China: Implications for magma mixing/mingling and mineralization

The origin of magmas with high H2O, S and metals that are linked to porphyry economic mineralization in continental collisional belts is controversial. In this paper, we studied the host porphyries and associated microgranular enclaves (MMEs) including gabbroic enclaves and dioritic enclaves from the large Beiya porphyry Au deposit in Jinshajiang-Ailaoshan mineralization belt, which provide first hand evidence of crust–mantle hybridization in the formation of fertile magma for porphyry deposit. Zircon LA-ICPMS U–Pb ages indicate coeval formation for the MMEs and the felsic porphyries in Eocene time (~36Ma). The MMEs show spheroidal shapes, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, which suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. One gabbroic enclave sample exhibits high MgO (5.6wt.%), high TiO2 (1.7wt.%), enriched in large-ion lithophile element (LILE) and depleted in high field strength element (HFSE), with negative Nb, Ta and Ti anomalies. This indicates that the gabbroic enclave is similar to those of the coeval potassic mafic rocks in the Western Yunnan, which originated from the enriched mantle metasomatized by the slab-derived fluids. The dioritic enclave exhibit both potassic and adakitic-like features, and are characterized by high contents of MgO (4.4–4.9wt.%), K2O (6.4–7.4wt.%), and compatible trace elements (e.g. Ni: 115–142ppm; Cr: 214–291ppm), as well as by high Sr/Y ratios. The dioritic enclave samples yield εHf (t) values (−1.9 to +3.6) similar to those of the host porphyries. Petrographic, elemental, and isotopic evidence suggests that the Beiya dioritic enclave were formed by mixing between potassic mafic and adakitc-like melts, derived from metasomatized lithospheric mantle and lower crust, respectively. In contrast, the Beiya host porphyries are characterized by high SiO2 (66.9–71.9wt.%) and Al2O3 (13.8–15.9wt.%) contents, high Sr/Y ratios (35–79), low compatible element contents (e.g. Ni=0.9–7.2ppm; Cr =1.0–8.2ppm), low Mg# values (0.19–0.35), positive LILE anomalies, marked negative high HFSE anomalies, negative εNd(t) values (−6 to −2), and high (87Sr/86Sr)i values (0.7071–0.7081). These features indicate that the host porphyries were mainly formed by partial melting of subduction-modified lower crust with a certain assimilation of mafic melts represented by MMEs. MMEs hosted in the Beiya porphyries, formed from a hydrous, mafic magma although they formed in a non-arc setting. High magmatic water content is probably related to an enriched mantle metasomatized by fluids from a Proterozoic subducted oceanic slab. High Cu (341–626ppm) and Au contents of the MMEs suggest they crystallized from a metal-rich mafic melt. Complex assemblages of pyrite–chalcopyrite–apatite–rutile with hornblende are observed in the least altered MME. We suggest that the mafic melt probably supplied part of water, and metal to the Beiya porphyry Au system.

Magma mixing and crust–mantle interaction in the Triassic monzogranites of Bikou Terrane, central China: Constraints from petrology, geochemistry, and zircon U–Pb–Hf isotopic systematics

Mafic microgranular enclaves (MMEs) are common in the monzogranites from the Yangba pluton in the Bikou Terrane of central China. Zircon LA-ICP-MS U–Pb dating yields ages of 208.7±0.7Ma and 209.3±0.9Ma for the host monzogranites, and 211.9±0.8Ma for the MMEs, indicating formation from coeval magmas. The field occurrence and textures including spheroidal shapes, transitional contacts, igneous mineral assemblages, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclase, indicate that the MMEs crystallized from mafic magma that was injected into and mingled with the host felsic magma. The host monzogranites are intermediate-felsic, metaluminous, and lack typical peraluminous minerals or alkaline mafic minerals, suggesting their I-type affinity. The monzogranites display depletion in high field strength elements, with Nb and Ta anomalies, and enrichment in large ion lithophile elements and light rare earth elements (LREE), with slightly negative Eu anomalies (Eu/Eu∗=0.72–0.93). The MMEs from the Yangba pluton are intermediate, metaluminous, and have higher K2O, Al2O3, Fe2O3T, MgO, Ni, and Cr contents than the pluton. The MMEs also display much higher total rare earth element (REE) concentrations and REE patterns that are sub-parallel to those of the host rocks, enrichments in Rb, Th, U, K, and LREE, and depletions in Ba, Nb, Ta, and Ti. These features are similar to those of the host rocks, indicating crust–mantle interaction during their petrogenesis.Zircons from the MMEs have highly variable εHf(t) values (−5.5 to +8.7) with corresponding two-stage Hf model ages (TDM2) of 1.14 to 1.42Ga, indicating they were derived from depleted mantle with crustal contamination. In contrast, zircons from host monzogranite show εHf(t) values ranging from −1.7 to +2.7, with TDM2 of 1.13 to 1.21Ga, corresponding to that of the basement rocks in the Bikou Terrane, which indicates that these granitic magmas were probably derived from the Neoproterozoic juvenile lower crust with minor contribution of Mesoproterozoic ancient lower crust.The integrated petrology and elemental and isotopic geochemistry suggest that the MMEs and host monzogranites were generated by the interaction of a granitic magma and a mafic magma in the lower crust. We envisage that Neoproterozoic subcontinental lithosphere mantle-derived mafic magma intruded the lower continental crust during Late Triassic, following the Late Permian to Early Triassic collision between the South China and North China Blocks.

Crustal thickening prior to 220 Ma in the East Kunlun Orogenic Belt: Insights from the Late Triassic granitoids in the Xiao-Nuomuhong pluton

The East Kunlun Orogenic Belt (EKOB) played an important role in plate tectonics, magma generation, and crustal evolution. Late Triassic granodiorites and their mafic micro-granular enclaves (MMEs) from Xiao-Nuomuhong in the EKOB were studied for geochemistry and geochronology to constrain their petrogenesis. Zircon LA-ICP-MS dating indicates that the Xiao-Nuomuhong granodiorites are coeval with their MMEs (∼222Ma). The granodiorites are high-K calc-alkaline rocks that are enriched in Rb, Th, U and LREE, and depleted in Cr, Ni and HFSE, with high Sr/Y ratios (82.2–85.3) and geochemically resemble the lower crust-derived adakites. The MMEs are also high-K calc-alkaline rocks, with high Al2O3 (16.8–18.8wt.%), low Mg# (30–40), Nb, Zr and Hf, with weak negative Eu anomalies (Eu/Eu#=0.8–0.9). We suggest the MMEs are mafic magmatic globules that were injected into the felsic host magma. The adakitic rocks from the Xiao-Nuomuhong pluton were generated by partial melting of thickened crust, while the primitive compositions of the MMEs were most likely from the lithospheric mantle beneath the EKOB. The Late Triassic Xiao-Nuomuhong pluton is important evidence that crustal thickening in the EKOB occurred prior to 220Ma. The pluton is interpreted as the result of mixing between thickened lower crust-derived melts and lithospheric mantle-derived mafic melts and the protracted magmatic response to the break-off of the Paleo-Tethys oceanic slab at ∼232Ma.

Coeval Felsic and Mafic Magmas in Neoarchean Calc-alkaline Magmatic Arcs, Dharwar Craton, Southern India: Field and Petrographic Evidence from Mafic to Hybrid Magmatic Enclaves and Synplutonic Mafic Dykes

Abstract We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30-40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (∼70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.

Mafic–felsic magma mixing limited by reactive processes: A case study of biotite-rich rinds on mafic enclaves

Mafic enclaves in felsic plutons are often used to argue that intermediate magmas are formed by mafic–felsic magma mixing, but the extent and nature of mixing remains unclear. Here, we examine biotite-rich rinds on mafic enclaves from the Cretaceous Bernasconi Hills Pluton in the Peninsular Ranges Batholith of southern California to gain insight into magma mixing processes. Rinds differ from the enclave interior and the host monzogranite in being more fine-grained and more mafic and potassic. Rinds are also 2–5 times more enriched in rare earth elements than the host monzogranite and up to 3 times more enriched than enclave interiors. These observations indicate that the rinds were not generated by isochemical quenching, binary mixing between enclave and host monzogranite, or in-situ magmatic differentiation. Instead, rinds appear to have been formed by chemical reaction between the solidified enclave and a hydrous K-rich residual melt or fluid formed after progressive crystallization and cooling of the host magma body, transforming amphibole in the enclave into biotite-rich rinds. Field observations show snapshots of biotite-rich rinds being eroded away and new rinds simultaneously forming on freshly eroded surfaces of enclaves, consistent with rinds being formed by chemical reaction instead of as quenching products. Deformation of enclaves is accommodated primarily by ductile attenuation of the thin rind while the enclave as a whole tends to rotate as a rigid body with minimal internal deformation other than localized brittle failure. A comparison of the aspect ratios and cross-sectional areas of mafic bodies in the pluton shows that those with high aspect ratios (indicating greater accumulated strain) are systematically more biotite-rich and have smaller cross-sectional areas than those with lower aspect ratios, which are amphibole-rich. These relationships not only confirm that biotite-rich lithologies are more deformable but also indicate that the high aspect ratio biotite-rich bodies (also known as schlieren) derive from small parent bodies, consistent with a derivation from eroding enclave rinds rather than from the enclave itself. Finally, geochemical and thermodynamic modeling indicates that the biotite-rich rinds formed when the host felsic magma had cooled to a low melt fraction state (F=0.15–0.3; 700–760 °C), suggesting that such reactions occur late in the lifespan of a magma body. Thus, mafic–felsic mixing may not be an efficient process for making intermediate magmas unless the magma body can reside at this low temperature range long enough to permit rind formation and subsequent deformation.

Origin of mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Zhejiang Province, Southeast China: Implications for magma mixing and crust–mantle interaction

The origin of mafic enclaves in granitoid plutons has long been a matter of debate. In this paper, we present detailed petrographic, mineralogical, geochemical, and SrNdHf isotopic data, UPb zircon dates, and field and petrological observations, for mafic microgranular enclaves (MMEs) and their host quartz monzonites from the Muchen pluton in Southeast China to evaluate their origins. LA–ICP–MS UPb dating of zircon yields crystallisation ages of 112.4±1.2Ma and 112.1±1.0Ma for the MMEs and host quartz monzonites, respectively, indicating their coeval formation during the late Early Cretaceous. Field and petrological observations, such as spheroidal shapes, back-veining, double enclaves, xenocrysts, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. Geochemically, the host monzonites are intermediate-acidic, metaluminous, alkaline, and K-rich. In contrast, the MMEs are relatively poor in Si and K. The host monzonites are enriched in Rb, Th and U, and depleted in Sr, P, Nb, Ta and Ti, and show moderate to strong europium depletions (δEu=0.12–0.60). These mineralogical and geochemical features classify the quartz monzonites as belonging to the I-type granitoids. The MMEs have broadly similar trace element signatures to those of the host monzonite, but are distinct in having relatively enriched Sr and P, more depleted Zr and Hf, and weak to moderate Eu depletions (δEu=0.43–0.93). Major and trace element data plotted versus isotopic data for the MMEs and the host quartz monzonites yield covariant arrays that result from magma mixing during their petrogenesis. The MMEs and the host quartz monzonites have similar initial 87Sr/86Sr ratios (ISr) of 0.7058–0.7070 and 0.7062–0.7065, respectively, and both have high εNd(t) values (−2.6 to +0.6 for MMEs; −3.2 to −2.4 for quartz monzonites). However, zircons from the MMEs have different εHf(t) values (−0.4 to +6.2) than the host quartz monzonites (−1.0 to +1.8), indicating that the MMEs and host granitoids largely crystallised from different magmas, providing direct evidence for mafic–felsic magma mixing processes. The zircon saturation geothermometer and Al-in-hornblende geobarometer show that the Muchen pluton crystallised at temperatures of 797–851°C and depths of 6–7km. The integrated petrology, and elemental and isotopic compositions suggest that the MMEs and host quartz monzonites were generated by mixing of depleted, mantle-derived, mafic magmas and felsic magmas produced by partial melting of crustal materials in an extensional setting.

Geochronology, petrogenesis and tectonic implications of the Zhongchuan granitic pluton in the Western Qinling metallogenic belt, China

The Zhongchuan district is an important component of the metallogenic belt in the Western Qinling. The Zhongchuan granite pluton occurring in the centre of the Zhongchuan metallogenic area has been poorly constrained, though the Triassic granite in Western Qinling has been well documented. In‐situ zircon U–Pb ages, Hf isotopic compositions and whole‐rock geochemical data are presented for host granite and mafic microgranular enclaves (MMES) from the Zhongchuan pluton, in order to constrain its sources, petrogenesis and tectonic setting of the pluton. The distribution of major, trace and rare earth elements apparently reflect exchange between the MMES and the host granitic rocks mainly due to interactions between coeval felsic host magma and mafic magma. The zircon U–Pb age of host granite (231.6 ± 1.5 to 235.8 ± 2.3 Ma) has overlapping uncertainty with that of the MMES (236.6 ± 1.3 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the MMES (εHf(t) = −13.4 to 4.0) is distinct from the host granite (εHf(t) = −15.7 to 0.0), indicating that both enriched subcontinental lithosphere mantle (SCLM) and crustal sources contributed to their origin. The zircons have two‐stage Hf model ages of 1064 to 1798 Ma for the host granite and 858 to 1747 Ma for the MMES. This suggests that the granitic pluton was likely derived from partial melting of a Late Mesoproterozoic crust, with subsequent interaction with the SCLM‐derived mafic magmas in tectonic affinity to the South China Block. Copyright © 2012 John Wiley & Sons, Ltd.

Granite, gabbro and mafic microgranular enclaves in the Gejiu area, Yunnan Province, China: a case of two-stage mixing of crust- and mantle-derived magmas

Geochronological, geochemical, whole-rock Sr–Nd, and zircon Hf isotopic analyses were carried out on the Jiasha Gabbro, mafic microgranular enclaves (MME) and host Longchahe Granite samples from the Gejiu area in the southeast Yunnan province, SW China, with the aim of characterizing their petrogenesis. Compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin. The Longchahe Granite largely comprises metaluminous quartz monzonite with distinctive K-feldspar megacrysts, but also contains a minor component of peraluminous leucogranite. The MME have spheroidal to elongated/lenticular shapes with sharp, crenulated and occasionally diffuse contacts with the host granite, which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma. Field observations, geochronology, geochemistry, Sr–Nd and zircon Hf isotopic compositions point to a complex petrogenesis for this granite–MME–gabbro association. Zircon 206Pb/238U ages determined by LA-ICP-MS for a mafic enclave, its host granite and the gabbro body are 83.1 ± 0.9 Ma, 83.1 ± 0.4 Ma and 83.2 ± 0.4 Ma, respectively, indicating coeval crystallization of these igneous rock units. Crystal fractionation processes can explain much of the compositional diversity of the Jiasha Gabbro. The geochemical features of the gabbro, such as high Mg# (up to 70) and Cr (up to 327 ppm), enrichment in LILEs (e.g., Rb, Ba, K2O) and LREEs, and depletion in HFSE (e.g., Nb, Ta, Ti), together with initial 87Sr/86Sr ratios of 0.708–0.709 and negative eNd(t) values (−5.23 to −6.45), indicate they were derived from a mantle source that had undergone previous enrichment, possibly by subduction components. The Longchahe Granite has a large range of SiO2 (59.87–74.94 wt%), is distinctly alkaline in composition, and has Sr–Nd–Hf isotopic compositions ((87Sr/86Sr)i > 0.712, eNd(t) = −6.93 to −7.62 and eHf(t) = −5.8 to −9.9) that are indicative of derivation from a crustal source. However, the most primitive rocks of Longchahe Granite are compositionally distinct from any feasible crustal melt. We interpret the spectrum of rock types of the Longchahe Granite to have formed via mixing between crustally derived peraluminous leucogranite magma and mantle-derived magma of similar heritage to the Jiasha Gabbro. We speculate that this mixing event occurred early in the magmatic history of these rocks at relatively high temperature and/or deep in the crust to allow efficient physical mixing of magmas. Saturation and accumulation of K-feldspar and zircon in the mixed magma is invoked to explain the megacrystic K-feldspar and elevated K2O and Zr content of some of the granitic rocks. A later episode of magma mixing/mingling is preserved as the MME that have geochemical and isotopic compositions that, for the most part, are intermediate between the granite and the gabbro. The MME are interpreted to be fractionated melts of mafic magma related to gabbro that were subsequently injected into the cooler, partly crystalline granitic magma. Mingling and mixing processes within the convectively dynamic upper crustal magma chamber resulting in a hybrid (MME) magma. During this second mixing episode, element interdiffusion, rather than bulk physical mixing, is interpreted to be the dominant mixing process.

Magma mingling and hybridization in the Kuh-e Dom pluton, Central Iran

The upper Eocene Kuh-e Dom granitoids in Central Iran comprise mafic microgranular enclaves ranging from a few centimeters to meters in size. The spherical to ellipsoidal enclaves consist of diorite, quartz-diorite, monzodiorite and quartz-monzodiorite whereas the more felsic host intrusions mainly comprise monzogranite and granodiorite. Most enclaves show sharp contacts with the host granites and distinct chilled margins in the small enclaves indicate rapid cooling after entering the felsic host magma. Disequilibrium textures include: (1) poikilitic and antirapakivi textures in feldspar megacrysts, (2) ocelli quartz rimmed by fine-grained mineral aggregates, (3) rounded and corroded plagioclase, (4) patchy plagioclase, (5) mafic clots, (6) blade biotite, (7) small lath-shaped plagioclase in larger tabular plagioclase, (8) acicular apatite, and (9) crenulated and cuspate contacts between enclaves and host rocks. These textures indicate mingling/mixing of two different magmas. Microprobe data show variable plagioclase compositions ranging between oligoclase and andesine in granodiorite, and between andesine to labradorite in the enclaves with normal zoning and oscillatory zoning. Geochemically, the enclaves are Na-rich whereas the host rocks are K-rich. In addition, the enclaves are depleted in LREE and LILE and enriched in HREE and Ti, P, Nb and Y, relative to their host granodiorites. Average initial Nd–Sr isotopic compositions for the host rocks are εNd (47Ma)=−5.09 and Sri=0.7068, compared to enclave Sri=0.7060 and εNd (47Ma)=−2.89. Fractionation of both enclaves and host rocks from the same melt cannot explain these petrological, geochemical and isotopic differences. Instead, enclaves and host rocks more likely originated from two separate magmas, which subsequently mingled and mixed. The mafic enclaves are modified mantle-derived melts while the granodioritic host rocks are partial melts of lower continental crust. The mafic enclave magma was at near liquidus conditions when injected into the granitic host melt, and the Kuh-e Dom intrusion reflects a hybrid product from mingling and partial (incomplete) mixing of these two melts.

The origin of mafic microgranular enclaves and their host granodiorites from East Kunlun, Northern Qinghai-Tibet Plateau: implications for magma mixing during subduction of Paleo-Tethyan lithosphere

Voluminous granodioritic magmatism is recorded in the East Kunlun, Northern Qinghai-Tibet Plateau. Here we present mineralogical, petrological, geochemical and Sr-Nd-Hf isotopic data and zircon U-Pb ages for the Naomuhun pluton and its numerous mafic microgranular enclaves (MMEs). Whole-rock geochemical data and regional geological studies indicate that this pluton consists of subduction-related high-K calc-alkaline metaluminous, I-type granodiorite. The MMEs have plagioclase xenocrysts and disequilibrium textures, such as oscillatory zoning and resorbed rims, indicating magma mixing. Compositions of plagioclase (An30-An49), amphibole (Mg# = 0.62 ~ 0.68), and biotite (Mg# = 0.52–0.56) of MMEs are similar to or very slightly different from equivalent minerals in the host granodiorites, suggesting nearly complete equilibration between the mafic-and felsic magmas. The zircon U-Pb age of the MMEs (263 ± 2 Ma) is identical, within analytical error, to that of the host granodiorites (261 ± 2 Ma). The MMEs have eHf(t) values of −6.83 to −3.15 (average = −4.68), whereas those of the granodiorites range from −9.00 to −3.20 (average −5.63), which is identical within analytical uncertainty. Combined with relatively homogeneous Sr-Nd isotopic compositions, we suggest the MMEs were derived from magma mixing, and their source is similar to an enriched mantle composition. The granodiorites have TDM2(Hf) model ages ranging from 1.49 to 1.86 Ga, consistent with the Nd model ages (TDM2), implying that the host magma was derived from Paleo- or Meso-proterozoic rocks, probably the Xiaomiao Group, which forms the basement of East Kunlun. We propose a model for magma formation and magma mixing in a subduction zone environment, in which subduction of an oceanic slab at ca. 260 Ma led to fluid metasomatism, inducing partial melting of an enriched lithospheric mantle to form the voluminous mafic magma. The mafic magma underplated the overlying lower crust, resulting in its partial melting to form felsic magma. The mafic magma then mixed with the felsic magma at lower crustal levels to form the MMEs by convective motion, or forceful injection into the host felsic magma. The MMEs and their host magma were then emplaced at a depth of ca. 12 km, where they crystallized at a temperature of ca. 700–770°C.

Enclaves in granitoids of north of Jonnagiri schist belt, Kurnool district, Andhra Pradesh: Evidence of magma mixing and mingling

Calc-alkaline, metaluminous granitoids in the north of Jonnagiri schist belt (JSB) are associated with abundant mafic rocks as enclave. The enclaves represent xenoliths of the basement, mafic magmatic enclaves (MME) and synplutonic mafic dykes. The MME are mostly ellipsoidal and cuspate shape having lobate margin and diffuse contact with the host granitoids. Sharp and crenulated contacts between isolated MME and host granitoids are infrequent. The MME are fine-grained, slightly dark and enriched in mafic minerals compare to the host granitoids. MME exhibits evidences of physical interaction (mingling) at outcrop scale and restricted hybridization at crystal scale of mafic and felsic magmas. The textures like quartz ocelli, sphene (titanite) ocelli, acicular apatite inclusion zone in feldspars and K-feldspar megacrysts in MME, megacrysts across the contact of MME and host and mafic clots constitute textural assemblages suggestive of magma mingling and mixing recorded in the granitoids of the study area. The quartz ocelli are most likely xenocrysts introduced from the felsic magma. Fast cooling of mafic magma resulted in the growth of prismatic apatite and heterogeneous nucleation of titanite over hornblende in MME. Chemical transfer from felsic magma to MME forming magma envisage enrichment of silica, alkalis and P in MME. The MME show low positive Eu anomalies whereas hybrid and host granitoids display moderate negative Eu-anomalies. Synplutonic mafic dyke injected at late stage of crystallising host felsic magma, display back veining and necking along its length. The variable shape, dimensions, texture and composition of MME, probably are controlled by the evolving nature and kinematics of interacting magmas.