Mafic-felsic Magma Research Articles

Polychronous and Multi-stage Growth History of Ladakh Batholith, Trans-Himalaya, through Mafic-Felsic Magma Mixing and Fractionation

Polychronous and Multi-stage Growth History of Ladakh Batholith, Trans-Himalaya, through Mafic-Felsic Magma Mixing and Fractionation

Dynamics of a subvolcanic magma chamber inferred from viscous instabilities owing to mafic-felsic magma interactions

Dynamics of a subvolcanic magma chamber inferred from viscous instabilities owing to mafic-felsic magma interactions

Peritectic assemblage entrainment and mafic–felsic magma interaction in the Late Oligocene–Early Miocene Karadağ Pluton in the Biga Peninsula, northwest Turkey: petrogenesis and geodynamic implications

Peritectic assemblage entrainment and mafic–felsic magma interaction in the Late Oligocene–Early Miocene Karadağ Pluton in the Biga Peninsula, northwest Turkey: petrogenesis and geodynamic implications

Understanding mafic-felsic magma interactions in a subvolcanic magma chamber using rapakivi feldspar: A case study from the Bathani volcano-sedimentary sequence, eastern India

The Ghansura Rhyolite Dome of the Bathani volcano-sedimentary sequence in eastern India originated from a subvolcanic felsic magma chamber that was intruded by volatile-rich basaltic magma during its evolution leading to the formation of a porphyritic andesite. The porphyritic andesite consists of rapakivi feldspars, which are characterized by phenocrysts of alkali feldspar mantled by plagioclase rims. Results presented in this work suggest that intimate mixing of the mafic and felsic magmas produced a homogeneous hybrid magma of intermediate composition. The mixing of the hot volatile-rich mafic magma with the relatively colder felsic magma halted undercooling in the subvolcanic felsic system and produced a hybrid magma rich in volatiles. Under such conditions, selective crystals in the hybrid magma underwent textural coarsening or Ostwald ripening. Rapid crystallization of anhydrous phases, like feldspars, increased the melt water content in the hybrid magma. Eventually, volatile saturation in the hybrid magma was reached that led to the sudden release of volatiles. The sudden release of volatiles or devolatilization event led to resorption of alkali feldspar phenocrysts and stabilizing plagioclase, some of which precipitated around the resorbed phenocrysts to produce rapakivi feldspars.

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

The effects of mafic-felsic magma interaction on magma diversity: insights from an early Paleozoic hornblendite-quartz monzonite suite in the South China block

Understanding the mechanisms responsible for the interplay between mafic and felsic magmas is the key to retrieving information on their sources, and characterizing the exchange of mass between them. In order to characterize compositional and mineralogical changes in the mafic end-member during mafic-felsic magma interaction and to better understand the nature of early Paleozoic intracontinental magmatism in the South China Block (SCB), a detailed study was conducted on an early Paleozoic hornblendite-quartz monzonite suite in the SCB. The amphibole phenocrysts in the hornblendite are zoned with respect to major and trace elements. From the brown core to the light-green rim, these amphibole phenocrysts display significant increases in Si, Mg, and Mn, coupled with abrupt decreases in Al, Ti, Na, K, and most of the trace elements, but only minor variations in Ca, Fe, Co, and Ni. The light-green matrix amphiboles in the hornblendite have similar compositions to the outer rim of amphibole phenocrysts (except Na). It is important to note that the amphibole grains in the quartz monzonite have significantly higher rare-earth element (REE) contents than the amphibole grains in the hornblendite. There is convincing evidence to support a significant transfer of incompatible elements (e.g., K, Na, LILE, LREE, U, and Th) from the felsic magma to the mafic magma, such as (1) the absence of high-Ca plagioclase in hornblendite, with the majority of feldspar grains being albite (Ab96–97) and orthoclase (Or94–96), and (2) uniform Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr = 0.7081–0.7098; eNd(t) = −6.8 to −6.3; weighted mean zircon eHf(t) = −8.0 to −7.4) for the hornblendite and quartz monzonite samples. It is, therefore, suggested that during mafic-felsic magma interaction, water was transferred from the quartz monzonite magma to the coeval hornblendite magma and promoted the formation of the amphibole crystals in the latter. The incompatible elements transferred from the quartz monzonite magma to the hornblendite magma were mainly incorporated into the late-crystallized anhedral phases in the hornblendite (e.g., orthoclase, sodic plagioclase, quartz, zircon, and apatite). This study suggests that water, which behaves as a supercritical fluid in most mafic-felsic magmas, may play a key role in the exchange of mass between mafic and felsic magmas, and this may be extended to the petrogenesis of biotite-amphibole aggregations in intermediate-felsic magmas.

Resolving tectonic settings of ancient magmatic suites using structural, geochemical and isotopic constraints: the example of the St Peter Suite, southern Australia

Reconstructing ancient tectonic environments is inherently difficult and requires the integration of many datasets. Here we attempt to reconstruct the tectonic setting of a Proterozoic magmatic rock suite from the Archean to Mesoproterozoic Gawler Craton, South Australia. We evaluate previous models of subduction or non-subduction tectonics for the St Peter Suite through field observations coupled with geochemical and isotopic data. Field observations indicate the presence of mafic magma chambers, felsic plutons and zones of mixing represented by multiphase plutons and zones of magma transfer. Syn-magmatic and solid-state deformation fabrics suggest magmatism was associated with a compressional tectonic regime, at least during some stages of magmatism. Zircon U–Pb geochronology suggests magmatism occurred between ca 1633 and ca 1608 Ma, an interval of ca 25 Myr. Mafic rocks are tholeiitic and enrichment in high field strength and light rare earth elements supports a modified mantle source region. Mafic rocks have εNd1620 Ma values around 0, and zircon εHf(t) values around 4, supporting minor crustal contamination. Felsic plutons have a geochemical signature similar to average continental crust, εNd1620 Ma between –3.7 and 0.4, and zircon Lu–Hf isotopic compositions that yield to εHf(t) values between –1.8 and 7.7, suggesting they were derived from fractionation of enriched tholeiitic parent magmas. A model for the formation of the St Peter Suite that is most consistent with the available data is one that places the magmatism in a broadly continental magmatic arc setting, albeit one in which little or no Archean crust was involved. Potentially this magmatic arc could have been built on a thinned, or hyper-extended continental margin. The integration of structural, geochemical and isotopic constraints has provided a more holistic view of the St Peter Suite petrogenesis than previously available.HighlightsEvaluation of models of subduction vs non-subduction tectonics for formation of Paleoproterozoic magmatic suiteEvidence for syn-magmatic compression and mafic–felsic magma minglingIsotopically juvenile (Nd--Sr--Hf) with mantle-like zircon δ18O valuesContinental arc setting proposed, possibly on hyper-extended continental margin

Tracing the styles of mafic-felsic magma interaction: A case study from the Ahvenisto igneous complex, Finland

Tracing the styles of mafic-felsic magma interaction: A case study from the Ahvenisto igneous complex, Finland

Synneusis: does its preservation imply magma mixing?

Abstract The Ghansura Felsic Dome (GFD) occurring in the Bathani volcano-sedimentary sequence was intruded by mafic magma during its evolution leading to magma mixing. In addition to the mafic and felsic rocks, a porphyritic intermediate rock occurs in the GFD. The study of this rock may significantly contribute toward understanding the magmatic evolution of the Ghansura dome. The porphyritic rock preserves several textures indicating its hybrid nature, i.e. that it is a product of mafic-felsic magma mixing. Here, we aim to explain the origin of the intermediate rock with the help of textural features and mineral compositions. Monomineralic aggregates or glomerocrysts of plagioclase give the rock its characteristic porphyritic appearance. The fact that the plagioclase crystals constituting the glomerocrysts are joined along prominent euhedral crystal faces suggests the role of synneusis in the formation of the glomerocrysts. The compositions of the glomerocryst plagioclases are similar to those of plagioclases in the mafic rocks. The results from this study indicate that the porphyritic intermediate rock formed by the mixing of a crystal-rich mafic magma and a crystal-poor felsic melt.

A Geochemical Overview of Mid-Archaean Metavolcanic Rocks from Southwest Greenland

The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.

Field evidence, mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61 wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57 wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.

Origin of the mafic microgranular enclaves (MMEs) and their host granitoids from the Tagong pluton in Songpan–Ganze terrane: An igneous response to the closure of the Paleo-Tethys ocean

The Songpan–Ganze terrane is mainly composed of a Triassic sedimentary sequence and late Triassic–Jurassic igneous rocks. A large number of plutons were emplaced as a result of tectono-magmatic activity related to the late stages of Paleo-Tethys ocean closure and ensuing collision. Granitoids and their hosted mafic enclaves can provide important constraints on the crust–mantle interaction and continental crustal growth. Mesozoic magmatism of Songpan–Ganze remains enigmatic with regard to their magma generation and geodynamic evolution. The Tagong pluton (209Ma), in the eastern part of the Songpan–Ganze terrane, consists mainly of monzogranite and granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (ca. 208–209Ma). The pluton comprises I-type granitoid that possesses intermediate to acidic compositions (SiO2=61.6–65.8wt.%), high potassium (K2O=3.2–4.1wt.%), and high Mg# (51–54). They are also characterized by arc-type enrichment of LREEs and LILEs, depletion of HFSEs (e.g. Nb, Ta, Ti) and moderate Eu depletions (Eu/Eu*=0.46–0.63). Their evolved zircon Hf and whole-rock Nd isotopic compositions indicate that their precursor magmas were likely generated by melting of old lower continental crust. Comparatively, the MMEs have lower SiO2 (53.4–58.2wt.%), higher Mg# (54–67) and show covariation of major and trace elements, coupled with field and petrographic observations, such as the disequilibrium textures of plagioclase and amphibole, indicating that the MMEs and host granitoids were originated from different magma sources but underwent mafic–felsic magma mixing process. Geochemical and isotopic data further suggest that the precursor magma of the MMEs was formed in the continental arc setting, mainly derived from an ancient metasomatized lithospheric mantle wedge.The Triassic granitoids from the Songpan–Ganze terrane show remarkable temporal–spatial-petrogenetic affinities to the counterparts of subduction zones in the Yidun and Kunlun arc terranes, plausibly support a double-sided subduction of the Paleo-Tethys ocean. The mixing mechanism for the formation of the Tagong pluton was likely associated with the break-off of a subducted slab of the Paleo-Tethys ocean, which triggered subsequent upwelling of hot asthenosphere beneath accreted arc fragments and induced lithospheric mantle-derived magmas suffice to underplate and mix with the lower crust-derived felsic magma. Collectively, the late Triassic igneous rocks record significant crustal growth and continental development as response to the final demise of the Paleo-Tethys ocean (ca. 210Ma), and marks the last episode of orogenic magmatism in the Songpan–Ganze terrane after which the region entered into post-orogenic phase of evolution.

Geochemistry and U–Pb SHRIMP zircon geochronology of microgranular enclaves and host granitoids from the South Khasi Hills of the Meghalaya Plateau, NE India: evidence of synchronous mafic–felsic magma mixing–fractionation and diffusion in a post-collision tectonic environment during the Pan-African orogenic cycle

Abstract Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km ( c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K 2 O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO 2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207 Pb/ 206 Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.

Geological Appraisal of Ladakh and Tirit Granitoids in the Indus-Shyok Suture Zones of Northwest Himalaya, India

Abstract: New field observations on granitoids and associated lithounits in some parts of Indus-Shyok Suture Zones have been documented in order to re-establish the geological relationships between various volcano-plutonic magmatic lithounits. Careful examination of outcrops and contact relationships between the various lithounits have pin-pointed the sequence of geological events. Field features of granitoids exposed along Leh-Saboo-Khardung_La suggest multiple pulses of mafic-felsic magma interactions (mingling to mixing) with almost 25% of the mafic to hybrid magma input in the evolution of the eastern part of Ladakh batholith. Along Khardung_La-Shyok-Diskit, thick sequence of volcanic lithounits is exposed, which dominantly consist of massive basaltic andesite, porphyritic andesite, dacite and rhyolite forming Khardung Formation. On the other hand Shyok Formation, dipping opposite to the Khardung Formation, composed predominantly of meta-sedimentary lithounits and subordinate amount of volcanic materials at present exposed level. Spectacular intrusive contacts of Ladakh granitoids with metavolcanics and meta-sedimentary country rocks of Shyok Formation near Diskit can be observed, which are manifested by ubiquitous xenoliths near the marginal parts. Although the nature of granitoid melt invasion into country-rocks was relatively winty, granitoid melt has produced leucogranite-pegmatite system because of devolatization and decompression effects. Frequent xenoliths of porphyritic andesite and dacite roof pendants are being reported in Tirit granitoids, which strongly suggest sub-volcanic emplacement of granitoid melt, extensive assimilation and roof collapse of overlying volcanic materials. It is more likely that the xenoliths hosted in Tirit granitoids belong to Shyok volcanics. It is suggested that multiple pulses of coeval mafic and felsic magmatism occurred extensively and emplaced at differential crustal levels.

Zircon U–Pb dating, geochemical and Sr–Nd–Hf isotopic characteristics of the Jintonghu monzonitic rocks in western Fujian Province, South China: Implication for Cretaceous crust–mantle interactions and lithospheric extension

Comprehensive petrological, in situ zircon U–Pb dating, Ti-in-zircon temperature and Hf isotopic compositions, whole rock geochemical and Sr–Nd isotopic data are reported for the Jintonghu monzonitic intrusions in the western Fujian Province (Interior Cathaysia Block), South China. The Jintonghu monzonitic intrusions were intruded at 95–96Ma. Their Sr–Nd–Hf isotopic compositions are similar to the coeval and nearby enriched lithospheric mantle-derived mafic and syenitic rocks, indicating that the Jintonghu monzonitic rocks were likely derived from partial melting of the enriched mantle sources. Their high Nb/Ta ratios (average 21.6) suggest that the metasomatically enriched mantle components were involved, which was attributed to the modification of slab-derived fluid and melt by the subduction of the paleo-Pacific Plate. The presence of mafic xenoliths, together with geochemical and isotopic features indicates a mafic-felsic magma mixing. Furthermore, the Jintonghu intrusions may have experienced orthopyroxene-, biotite- and plagioclase-dominated crystallization. Crust–mantle interaction can be identified as two stages, including that the Early Cretaceous mantle metasomatism and lithospheric extension resulted from the paleo-Pacific slab subduction coupled with slab rollback, and the Late Cretaceous crustal activation and enhanced extension induced by dip-angle subduction and the underplating of mantle-derived mafic magma.

Evidence for hybridisation in the Tynong Province granitoids, Lachlan Fold Belt, eastern Australia

ABSTRACTThe role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.

Magma mixing/mingling in Salmas granodiorite, NW Iran: evidence from mafic microgranular enclaves

The interaction of mafic–intermediate and felsic rocks of the Salmas plutonic rocks produced mixed rocks (granodiorites) which contain mafic microgranular enclaves (MMEs). Enclaves ranging from a few millimeters to centimeters in size, and from ellipsoidal to rounded in shape. Based on both field observation and mineralogical compositions, MMEs are composed of quartz diorite whereas the felsic host rocks comprise mainly granodiorite. MMEs are characterized by a microporphiritic texture and revealed some types of microscopic textures, e.g., prismatic-cellular plagioclase with spike zones and rounded plagioclase megacrysts, blade-shaped biotite and acicular apatite. The host rocks show textures such as oscillatory- and reversely zoned plagioclase with spike zone. Compositions of plagioclases (An41 to An48) of MMEs are similar to those of host rocks (An38 to An45) which suggest partial to complete equilibration during mafic–felsic magma interactions. The individual petrographic and microstructural textures and mineral chemistry similarities between the MMEs and their host rocks and diorites indicate that the enclaves are of mixed origin and most probably formed by interaction of lower crust magma (granitic melt) and evolved mantle-derived magmas (diorites).

Crust/mantle interaction during the construction of an extensional magmatic dome: Middle to Late Jurassic plutonic complex from western Liaoning, North China Craton

Abstract Differentiating magmatic doming and low-angle normal faulting remains critical for fully understanding the thermal, mechanical and chemical evolution of continental landmasses under extension. This zircon U–Pb dating and geochemical study documents two Middle to Late Jurassic batholiths (Lushan and Haitangshan) from the Yiwulushan range of western Liaoning, North China Craton. They consist of a variety of lithologies including gabbro, diorite, granodiorite, monzogranite, together with microgranular magmatic enclaves (MME) and mafic dykes. Synthesizing petrologic, elemental, whole-rock Sr–Nd and zircon Hf isotopic data leads to the characterization of multiple mafic and felsic end-members and their concomitant interaction in building the magmatic dome. A subduction-related metasomatized lithospheric mantle source is fingerprinted by the gabbroic to dioritic rocks with enriched large ion lithophile elements, depleted high field strength elements and heterogeneous isotopic compositions ( 87 Sr/ 86 Sr i = 0.70541 to 0.70577, e Nd (t) = − 1.78 to − 5.54 and zircon e Hf (t) = − 6.0 to 8.1). One felsic magma end-member of ancient mafic lower crustal parentage is discernable from adakitic granites with high Sr/Y and evolved isotopic composition ( 87 Sr/ 86 Sr i = 0.70533 to 0.70792, e Nd (t) = − 18.8 to − 21.7, zircon e Hf (t) = − 18.5 to − 28.8), whereas another felsic magma end-member of newly underplated crustal heritage manifests itself from some monzogranites with non-adakitic elemental affinity and juvenile isotopic composition ( 87 Sr/ 86 Sr i = 0.70429 to 0.70587, e Nd (t) = − 4.47 to − 5.87, zircon e Hf (t) = 4.3 to 1.3). Hybridization processes between mantle-derived mafic magma and ancient crustal-derived felsic magma result in the formation of MME-bearing granodiorites with intermediate isotopic signatures ( 87 Sr/ 86 Sr i = 0.70491 to 0.70499, e Nd (t) = − 15.3 to − 15.8, zircon e Hf (t) = − 12.7 to − 17.4). Subsequent fractional crystallization of the hybridized magmas endows the differentiated monzogranites with low Sr/Y and highly evolved isotopes ( 87 Sr/ 86 Sr i = 0.70496 to 0.70605, e Nd (t) = − 16.0 to − 18.7, zircon e Hf (t) = − 14.3 to − 21.5). Apart from distinguishing Middle–Late Jurassic extensional magmatic doming from Early Cretaceous detachment faulting, this complex mafic–felsic magma association encapsulates a multi-level crust/mantle interaction leading to lithospheric thinning and concomitant crustal architectural reorganization in the Yanshan belt during the Late Mesozoic. Near-synchronization of a two-stage extensional pattern in the Yanshan belt and even across NE continental Asia accords well with gravitational collapse and convective removal of lithospheric mantle within an evolved post-collisional to within-plate extensional regime.

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.