Mafic Enclaves Research Articles

Batch-wise assembly and zoning of a tilted calc-alkaline batholith: Field relations, timing, and compositional variation

The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 158.99 ± 0.17 and 159.22 ± 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 ± 0.46 to 158.21 ± 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystal-liquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 ± 0.20 to 158.30 ± 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (

An exotic Mesoarchean microcontinent: The Coorg Block, southern India

Sandwiched between the Dharwar Craton in the north and the Neoarchean–Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite–trondhjemite–granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of all the major lithologies from this block. Computation of metamorphic P–T conditions from mineral chemical data shows consistent granulite-facies P–T conditions of 820–870°C and up to 6kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean 207Pb/206Pb ages of 3153.4±9 to 3184.0±5.5Ma for syenogranites, 3170.3±6.8Ma for biotite granite, 3275±5.1Ma for trondhjemite, 3133±12 to 3163.8 ±6.9Ma for charnockites, 3156±10 to 3158.3±8.2 for mafic enclaves, 3161±16Ma for diorite and 3173±16Ma for felsic volcanic tuff. An upper intercept age of 3363±59Ma and a lower intercept age of 2896±130Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from >3.3 to 3.1Ga was immediately followed by metamorphism at ca. 3.0 to 2.9Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5Ga, 2.0Ga, 0.8Ga and 0.55Ga associated with various subduction–accretion–collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.

Grain to outcrop-scale frozen moments of dynamic magma mixing in the syenite magma chamber, Yelagiri Alkaline Complex, South India

Magma mixing process is unusual in the petrogenesis of felsic rocks associated with alkaline complex worldwide. Here we present a rare example of magma mixing in syenite from the Yelagiri Alkaline Complex, South India. Yelagiri syenite is a reversely zoned massif with shoshonitic (Na2O + K2O=5–10 wt.%, Na2O/K2O = 0.5–2, TiO2 <0.7 wt.%) and metaluminous character. Systematic modal variation of plagioclase (An11–16 Ab82–88), K-feldspar (Or27–95 Ab5–61), diopside (En34–40Fs11–18Wo46–49), biotite, and Ca-amphibole (edenite) build up three syenite facies within it and imply the role of in-situ fractional crystallization (FC). Evidences such as (1) disequilibrium micro-textures in feldspars, (2) microgranular mafic enclaves (MME) and (3) synplutonic dykes signify mixing of shoshonitic mafic magma (MgO = 4–5 wt.%, SiO2 = 54–59 wt.%, K2O/Na2O = 0.4–0.9) with syenite. Molecular-scale mixing of mafic magma resulted disequilibrium growth of feldspars in syenite. Physical entity of mafic magma preserved as MME due to high thermal-rheological contrast with syenite magma show various hybridization through chemical exchange, mechanical dilution enhanced by chaotic advection and phenocryst migration. In synplutonic dykes, disaggregation and mixing of mafic magma was confined within the conduit of injection. Major-oxides mass balance test quantified that approximately 0.6 portions of mafic magma had interacted with most evolved syenite magma and generated most hybridized MME and dyke samples. It is unique that all the rock types (syenite, MME and synplutonic dykes) share similar shoshonitic and metaluminous character; mineral chemistry, REE content, coherent geochemical variation in Harker diagram suggest that mixing of magma between similar composition. Outcrop-scale features of crystal accumulation and flow fabrics also significant along with MME and synplutonic dykes in syenite suggesting that Yelagiri syenite magma chamber had evolved through multiple physical processes like convection, shear flow, crystal accumulation and magma mixing.

Late Cretaceous magmatism in Mamba area, central Lhasa subterrane: Products of back-arc extension of Neo-Tethyan Ocean?

Cretaceous magmatism in southern Lhasa subterrane, Tibetan plateau has been investigated for many years and a series of models have been proposed to illustrate their petrogenesis and geodynamic implications. But rare work has been done on the Cretaceous magmatism in central Lhasa subterrane. Here we report the petrology, zircon in situ U–Pb geochronology, Hf isotopes, trace element, and whole-rock elements and Sr–Nd isotopic geochemical data of the host granodiorites, and gabbroic and dioritic enclaves in Mamba area, central Lhasa subterrane. Zircon U–Pb dating for a Mamba host granodiorite yields a crystallization age of ~84Ma, with in situ Hf isotopic analyses for 18 spots of the same zircons of εHf (t) ranging from −7.5 to −0.3. A dioritic enclave (85.2Ma) is coeval with the host granodiorite and shows similar zircon Hf isotopic compositions (εHf (t)=−4.0 to +0.2). Mamba granodiorites (SiO2=66.6–67.5wt.%) and dioritic enclaves (SiO2=53.9–57.6wt.%) are high-K calc-alkaline, and a gabbroic enclave is shoshonitic (K2O=2.81%). All these samples are metaluminous, and enriched in large ion lithophile elements (LILEs, such as Rb, Ba, K, U, Th) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and Zr). The host granodiorites are enriched in light rare earth elements (REEs), depleted in heavy REEs with weakly negative Eu anomalies (δEu=0.86–0.88), with high Al2O3 (15.0–15.7wt.%), high Sr/Y ratio (58.1–68.3) and Sr (680–755ppm), and low Y (10.8–13.0ppm) abundance, suggesting adakitic affinities. Mamba adakitic granodiorites, gabbroic and dioritic enclaves exhibit homogeneous Sr isotopes ((87Sr/86Sr)i=0.7066–0.7067, 0.7073, and 0.7067, respectively) and Nd isotopes (εNd(t)=−5.7 to −4.4, −4.0, and −3.6, respectively). These geochemical features allowed us to conclude that the adakitic host granodiorites and mafic (gabbroic–dioritic) enclaves were derived from magma mixing between ancient thickened lower crust and enriched fluid-metasomatized mantle. The distance between Mamba and the suture zone was more than 200km when the intrusives emplaced at ~85Ma, which implies that these rocks cannot be resulted from the mid-ocean ridge subduction. Combining of the intra-plate environment indicated by the gabbroic enclave of this study, the presence of the coeval bimodal igneous rocks in the similar latitude in central Lhasa subterrane, and other records in late Cretaceous sedimentary basin, the Mamba ~85Ma magmatism were attributed to the back-arc extension of Neo-Tethyan Ocean.

Pre-eruptive Magmatic Conditions at Augustine Volcano, Alaska, 2006: Evidence from Amphibole Geochemistry and Textures

Variations in the geochemistry and texture of amphibole phenocrysts erupted from Augustine Volcano in 2006 provide new insights into preand syn-eruptive magma storage and mixing. Amphiboles are rare but present in all magma compositions (lowto high-silica andesites) from the 3 month long eruption. Unzoned magnesiohornblende in the highand low-silica andesites exhibit limited compositional variability, relatively high SiO2 (up to 49·7 wt %), and relatively lowAl2O3 (511·1wt %). Intermediate-silica andesites and quenched mafic enclaves contain amphiboles that vary in composition (e.g. SiO2 40·8^48·9 wt %, Al2O3 6·52^15·2 wt %) and classification (magnesiohornblende^magnesiohastingsite^ tschermakite). Compositional variation in amphibole is primarily controlled by temperature-dependent substitutions. Both highand low-silica andesites represent remnant magmas that were stored in the shallow crust at 4^8 km depth, remaining distinct owing to a complex subsurface plumbing system. Intermediate-silica andesites and quenched mafic inclusions represent pre-eruptive hybrids of resident highand low-silica andesite magmas and an intruding basalt. Amphiboles in explosive phase high-silica andesites are largely euhedral and unreacted, consistent with the high magma flux rates from depth during this phase (up to 13 800 m s). Phenocrysts from the other lithologies have reaction rims that range from 1 to41000 m in thickness. Reaction rim microlite sizes correlate with reaction rim thicknesses. Reaction rims550 m thick contain microlites 1^10 m in length whereas reaction rims480 m thick contain microlites 10^100 m in length. Differentiating between heatingand decompression-induced amphibole reaction rim formation is problematic because of a lack of experimental constraints.We attempt a new approach to assessing reaction rim formation, based on a kinetic theory of crystal nucleation and growth, in which the differences in reaction rim textures represent different degrees of amphibole disequilibrium. Large crystals and low number densities suggest relatively lower levels of disequilibrium resulting in growth-dominated crystallization. Smaller crystals and larger number densities are indicative of higher nucleation rates and a high driving force.

Transitional I S type characteristic in the Main Range Granite, Peninsular Malaysia

The dominantly Triassic Main Range Granite of Peninsular Malaysia that occurs west of the Paleo-Tethyan Bentong–Raub suture zone was regarded exclusively as an S-type granite. The Main Range dominantly biotite granites are of batholithic proportion and host one of the world’s largest tin provinces. The S-type characteristics include high initial 87Sr/86Sr isotope ratios (>0.710), a narrow range in silica, presence of ilmenite and occasional cordierite and andalusite, and the presence of pelitic or quartzose meta-sedimentary xenoliths. However, the present review shows that the Main Range Granites also have many features that are more characteristic of I-type granites such as the very large scale of plutonism, the presence of primary titanite and amphibole, occurrence of hornblende-bearing mafic enclaves, increasing peraluminosity towards the more differentiated end-members of the suite and decreasing P2O5 with increasing SiO2 contents. The moderately peraluminous nature of the bulk Main Range Granite, without containing cordierite, Fe–Mg garnet or sillimanite, is consistent with derivation from a meta-sedimentary protolith that was itself undersaturated with respect to Al2SiO5.

Petrogenesis of the Üçkapılı Granitoid and its Mafic Enclaves in Elmalı Area (Niğde, Central Anatolia, Turkey)

Abstract:The Late Cretaceous Üçkapılı Granitoid including mafic microgranular enclaves intruded into metapelitic and metabasic rocks, and overlain unconformably by Neogene ignimbrites in the Niǧde area of Turkey. It is mostly granite and minor granodiorite in composition, whereas its enclaves are dominantly gabbro with a few diorites in composition. The Üçkapılı Granitoid is composed mainly of quartz, K‐feldspar, plagioclase, biotite, muscovite and minor amphibole while its enclaves contain mostly plagioclase, amphibole, minor pyroxene and biotite. The Üçkapılı Granitoid has calcalkaline and peraluminous (A/CNK= 1.0–1.3) geochemical characteristics. It is characterized by high LILE/HFSE and LREE/HREE ratios ((La/Lu)N= 3–33), and has negative Ba, Ta, Nb and Eu anomalies, resembling those of collision granitoids. The Üçkapılı Granitoid has relatively high 87Sr86Sr(i) ratios (0.711189–0.716061) and low εNd(t) values (−5.13 to −7.13), confirming crustal melting. In contrast, the enclaves are tholeiitic and metaluminous, and slightly enriched in LILEs (K, Rb) and Th, and have negative Ta, Nb and Ti anomalies; propose that they were derived from a subduction‐modified mantle source. Based on mineral and whole rock chemistry data, the Üçkapılı granitoid is H‐(hybrid) type, post‐collision granitoid developed by mixing/mingling processes between crustal melts and mantle‐derived mafic magmas.

Double injection events of mafic magma into supersolidus Yucheon granites to produce two types of mafic enclaves in the Cretaceous Gyeongsang Basin, SE Korea

Petrographic and geochemical features of the Cretaceous Yucheon granites and their mafic microgranular/magmatic enclaves (MMEs), SE Korea, reveal that the MMEs originated from magma mixing. Mesoscopic and microscopic features indicate that mechanical mixing operated heterogeneously to produce the MMEs with a wide range of sizes and textures. Chemical compositions of amphibole, biotite, and plagioclase rims of both the MMEs and host granites are almost identical, indicating that chemical homogenization took place to some extent after the mechanical mixing. Plagioclase cores, however, have various compositions depending on the host rocks and/or sampling locations, suggesting their sluggish re-equilibration. The MMEs are divided into Type A (low TiO2, very fine-grained, chilled margins) and Type B (high TiO2, fine- to medium-grained, no chilled margins). The lower TiO2 MMEs cooled more rapidly and interacted with granitic magma for a shorter period of time than the higher TiO2 MMEs. Additionally, the former are less enriched in HREEs than the latter. Zoned plagioclase has two zones of increased An content. These features are indicative of double injection events of mafic magma. A previous model explains the magma mixing as resulting from the generation of a slab window due to Kula-Pacific ridge subduction. The model cannot, however, explain the eastward younging of the granites in Korea, necessitating a new, more elaborate model of Cretaceous geodynamics and magmatism in East Asia.

The giant Dexing porphyry Cu–Mo–Au deposit in east China: product of melting of juvenile lower crust in an intracontinental setting

The Dexing porphyry Cu–Mo–Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu–Mo–Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172–170 Ma) of the felsic magmatism and the mineralization (171 ± 1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40–60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform e Nd(t) values from −0.9 to +0.6, and wide (87Sr/86Sr)i range between 0.7046 and 0.7058 partially overlapping with the Sr–Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive e Hf(t) values (3.4 to 6.9), and low δ18O values (4.7 to 6.3 ‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by Proterozoic slab-derived fluids, and supplied a part of Cu, Au, and S for the Dexing porphyry system during their injection into the felsic magmas. The 171 ± 1 Ma magmatic-hydrothermal event at Dexing is contemporaneous with the mid-Jurassic extension in the SCC, followed by 160–90 Ma arc-like magmatism in southeastern China. With respect to the tectono-magmatic evolution of the SCC, the emplacement of Cu-bearing porphyries and the associated Cu mineralization occurred in response to the transformation from a tensional regime, related to mid-Jurassic extension, to a transpressional regime, related to the subduction of the Paleo-Pacific oceanic lithosphere.

Magmatic emulsion texture formed by mixing during extrusion, Rauðafell composite complex, Breiðdalur volcano, eastern Iceland

The Rauðafell composite complex is part of the Neogene Breiðdalur volcano, eastern Iceland and is composed of a composite feeder dyke, a vent structure and a composite flow. The two end-members of the composite complex are rhyolite and basalt, and both are rich in plagioclase macrocrysts: bytownite in basalt and oligoclase in rhyolite. The rhyolite also includes ferroaugite macrocrysts. The mixed rocks are separated in three textural groups related to mixing proportions. When the basaltic end-member is dominant, a hybrid texture with a homogeneous matrix is observed and the only evidence of mixing is the presence of antecrysts of both end-members. When the basaltic end-member represents c. 65 to 30 % of the mixture, we observe emulsion textures composed of finely co-mingled basalt and rhyolite. The difference between these two textural expressions of mixing is due to effects of diffusion. The third texture shows mafic enclaves suspended in a rhyolitic matrix. In these rocks, the proportion of the basaltic end-member is inferior to 30 %, implying that the basalt froze solid in contact with the rhyolite. Zoning of plagioclase shows that the mixing processes are driven initially by highly efficient micro-mingling; the emulsification is possibly a result of compositional gradient stresses (Korteweg stress) between miscible basalt and rhyolite. This is followed by chemical diffusion (hybridisation) and tend to protect antecrysts from reaction with the primitive magmas. When antecrysts originated in the evolved magma, they undergo dissolution due to thermal disequilibrium during mingling and chemical disequilibrium during hybridisation. We argue that such mixing processes are important in producing intermediate rocks in Iceland and elsewhere that shows only the chemical attributes of an origin by mixing. The preservation of emulsion textures is rare and highly dependent on cooling history.

Mafic microgranular enclaves in Late Paleozoic granitoids in the Burgasy quartz syenite massif, western Transbaikalia: Composition and petrogenesis

The paper presents original data on the inner structure, mineralogy, and geochemistry of the Late Paleozoic Burgasy quartz syenite massif in western Transbaikalia and mafic microgranular enclaves (MME) in its rocks. The composition of the mafic microgranular enclaves is close to that of phase-1 monzonitoids of this pluton, but the enclaves are not xenoliths of these rocks but were produced by the crystallization of an individual portion of dispersed hybridized basalt melt. The basaltoid nature of the enclaves follows, first of all, from the relict assemblage of calcic plagioclase (An73–60) and clinopyroxene and from the magmatic dolerite and microgabbro textures of the rocks. The monzonitoid composition of the enclaves was caused by hybridism, which was responsible for the crystallization of quartz, potassic feldspar, and sodic plagioclase due to the introduction of silica, potassium, and some other components. Hybridism was restricted to a boundary crystallization layer in the deep portion of the magmatic chamber (near its bottom). The scatter of the enclaves throughout the whole volume of the pluton is explained by the density inversion of the hybrid layer and material transfer by convective flows. The mafic enclaves crystallized from basaltic melt of within-plate geochemical type. In spite of intense hybridism, the enclaves preserved typical compositional signatures of mafic magma related to the generation of granites in western Transbaikalia in the Late Paleozoic. The basaltoid nature of the mafic enclaves of the Burgasy Massif testifies that magma was simultaneously generated in the mantle and crust during the development of the Late Paleozoic province in the area.

Multi-stage granitic magmatism during exhumation of subducted continental lithosphere: Evidence from the Wulong pluton, South Qinling

Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30Ma is recorded from the 225–235Ma quartz diorite on the pluton margin, the ca. 218Ma granodiorite in the intermediate zone, and the ca. 207Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with 87Sr/86Sr(i) of 0.7044–0.7062, unradiogenic Nd (εNd(t) values of −6.1 to −3.0, Nd model ages of 1.1–1.3Ga, and moderately radiogenic Pb compositions (206Pb/204Pb(i)=17.500–17.872, 207Pb/204Pb(i)=15.513–15.549, 208Pb/204Pb(i)=37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with εHf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973ppm), high Sr/Y ratios (mostly >60) and negligible Eu anomalies (Eu/Eu*=0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17ppm), HREE (Yb=0.95–1.7ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet+amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO2 (60.65–63.98wt.%) and high TiO2 (0.39–0.86wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO2 (69.16–70.82wt.%), high Al2O3 (15.33–16.22wt.%) and A/CNK values (mostly >1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO2 (67.68–70.29wt.%), low TiO2 (mostly <0.35wt.%) and high Sr/Y ratios of 86–151.

Zircon U–Pb ages, trace elements and Nd–Hf isotopic geochemistry of Guyang sanukitoids and related rocks: Implications for the Archean crustal evolution of the Yinshan Block, North China Craton

The Yinshan Block is a unique coherent Neoarchean basement terrane of the Western Block of the North China Craton. In this paper, three Archean plutons, namely the Dajitu, Banshentu and Haolaigou plutons, have been investigated. Based on the field observations, the major difference among these plutons is that many ultramafic–mafic enclaves are enclosed in the Dajitu and Banshentu plutons, whereas none occurs in the Haolaigou pluton. Geochemically, the Dajitu and Banshentu plutons are characterized by low SiO2 contents (56–67wt%), high MgO (1.9–7.1wt%), Cr (10–947ppm), Ni (2–274ppm) and Mg# (44–64) contents, negative Nb, Ta and Ti anomalies, high large ion lithophile elements (LILEs), with no or slightly negative Eu anomalies, similar to a typical Archean sanukitoid. New geochronological data indicate that the Dajitu and Banshentu plutons were emplaced at 2523±7Ma and 2480±20Ma, with inherited ages of 2606±19Ma and 2647±19Ma, respectively. In contrast, the Haolaigou pluton shows high SiO2 (68–71wt%), MgO, Mg# (46–55), Cr, and Ni contents, negative Nb, Ta and Ti anomalies, and high LILEs, with strongly positive Eu anomalies, high Sr and Ba concentrations and Sr/Y and La/Yb ratios, and low Y and HREEs abundances. It was emplaced at 2466±15Ma, with an inherited age of 2638±21Ma. The whole-rock ɛNd(t) and zircon ɛHf(t) values of the Haolaigou pluton are positive. The whole-rock TDM(Nd) and zircon TDM1(Hf) values range between 2.6 and 2.8Ga with a peak at ∼2.7Ga, indicating that the melt was generated by the partial melting of a subducted slab interacting with the overlying mantle wedge. The Dajitu pluton has Nd and Hf isotope characteristics identical to those of the Haolaigou pluton, implying that the two plutons have similar sources. It should be noted, however, that the Dajitu pluton displays sanukitoid characteristics, which are the result of interaction with ultramafic–mafic enclaves. The whole-rock ɛNd(t) and zircon ɛHf(t) values of the Banshentu pluton are mostly negative. The whole-rock TDM(Nd) and zircon TDM1(Hf) values of the pluton range between 2.7 and 2.9Ga with a peak at ∼2.8Ga, indicating that the pluton was generated from the melting of a lower crust. The sanukitoid characteristics of the pluton resulted from interactions with an ultramafic enclave. Taken together, these results with the geochronological geochemical data for other Archean rocks in this area, suggest that the Yinshan Block experienced significant crustal growth at 2.8–2.6Ga and further crustal reworking that led to strong magmatism at ∼2.5Ga.

Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)

Li, Be, B and δ7Li SIMS analyses of plagioclase phenocrysts from the 1040–1941 Niki dacite lava (Nea Kameni, Santorini, Greece) exhibit varied processes. From their anorthite contents alone, the crystals may be segregated into four main types: type-N shows the normal decline in An during crystallisation (An62–40); type-O has only oscillatory zoning accompanied by resorption surfaces (An58–39); type-C is complex with high-An cores (subtype C1: An64–58, subtype C2: An88–73) and normal rims (An55–42). Type-A plagioclase with high An content (An92–82) is found within mafic enclaves. On the basis of their Li concentrations, type-O crystals may be subdivided into subtype O1 with flat Li concentration profiles and subtype O2 with decreasing Li concentration from core to rim. The concentrations of Be and B of all four types show a negative correlation with anorthite content (An), but Li concentration profiles differ amongst the different plagioclase types. Types N and O1, and the cores of type-C, are equilibrated in Li concentration. Types O2 and A, and the mantles of type-C display an initial enrichment in Li, probably from volatile influx into the melt. Consistent with the propensity towards equilibrium with the melt, these crystals display dramatic rim-ward declines in Li concentration. All analysed plagioclase crystals, except for the xenocrystic type-A, have nearly the same Li, Be and B concentrations at their rims. These coincide with the composition of plagioclase microlites in the groundmass, thereby affording estimates of plagioclase-melt partitioning for the light elements: K Li = 0.19–0.28, K Be = 0.24–0.38 and K B = 0.007–0.009. δ7Li profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to ~39 ‰, revealing preferential kinetic diffusion. This may have been driven by Li loss from the melt, most likely through degassing during decompression, perhaps in the course of magma ascent to subsequent eruption. Considering the rapid diffusion of Li in plagioclase, in situ phenocryst analyses may yield useful information about processes leading up to, or even causing, eruptions.

Hercynian plutonic rocks of Voras Mountain, Macedonia, Northern Greece: their structure, petrogenesis, and tectonic significance

We studied the structural, petrological, and geochemical characteristics, as well as the geotectonic setting, of the Hercynian (300 ± 3 Ma) Voras plutonic rocks intrusive into the lower part of the Pelagonian nappe pile (East Pelagonian Zone, EPZ). These rocks are compared with the neighbouring Hercynian Varnountas and Kastoria plutons intruding the tectonic upper Pelagonian part (Korabi West Pelagonian Zone, KoWPZ). Based on modal and chemical compositions, four rock-types can be distinguished for the Voras ploutonic rocks: (1) hornblende-biotite granodiorite to granite (HbBtGrd), (2) biotite granite (BtGr), (3) leucogranite (LGr), and (4) mafic microgranular enclaves (MMEs) occurring typically in HbBtGrd and BtGr. Aplites intrude HbBtGrd and BtGr whereas xenoliths are rare. The MMEs are metaluminous, while all the other rock-types are slightly peraluminous. Crystallization pressures range from 2.4 to 2.9 kbar for the HbBtGrd and 3.0 kbar for the MME. Field observations, chemical, mineralogical, and petrographical data suggest that theMME and HbBtGrd + BtGr formed as a result of a two-step evolution process. In the first step, a mantle-sourced basic magma with composition similar to the more basic MMEs fractionated (F = 0.40, 60% crystallization) and concurrently mixed with an acid magma, of composition similar to the more acid BtGr. The crystallized mineral assemblage is Qtz38.27Pl27.83Hb14.84Bt10.61Kf6.13Ap2.30Zrn0.02. The process evolved with a low r-value (r = 0.3) giving the more basic HbBtGrd. In the second step, evolved magma (more basic HbBtGrd) fractionated, while simultaneously mixing with the same acid magma, but with higher r-value (r = 0.8), giving the more evolved HbBtGrd and the BtGr after 50% crystallization (F = 0.50) to the phase assemblage Qtz30.07Pl27.75Hb5.59Bt3.33Kf23.26Tit5.86Ap1.91Zrn0.04Mt2.19. We interpret the evolution of the LGr through fractional crystallization; it formed by partial melting of gneisses or felsic charnokites and granoulites. The less evolved MME (basic end-member) had a mantle origin, whereas for the more evolved BtGr (acid end-member) we favour partial melting of gneisses or mafic charnokites. Detailed structural analysis shows a strong, polyphase Alpine deformation which affected the Hercynian Voras ploutonic rocks and is thoroughly imprinted on the host Pelagonian metamorphic basement rocks. No evidence of relict Hercynian structures has been recognized, possibly due to intense reworking by the younger Alpine deformation. We have identified five tectonic events (D1–D5) from the Late Jurassic to recent, that evolved progressively from ductile, synmetamorphic (D1, D2) to semi-ductile (D3), and finally brittle (D4, D5) conditions. Voras plutonic rocks in the EPZ, and Varnountas and Kastoria plutons in the KoWPZ, have similar major and trace element geochemistries, as well as structural evolution; they coevally intruded (Carboniferous) the Pelagonian continent as a single unit. They show similar crystallization pressures and mantle contributions for their genesis; both are related to a volcanic arc geotectonic setting, associated with the subduction of the Palaeotethys Ocean beneath the Pelagonian continental fragment, the latter possibly of Gondwana origin.

The fast evolution of a crustal hot zone at the end of a transpressional regime: The Saint-Tropez peninsula granites and related dykes (Maures Massif, SE France)

Granite and dolerite occurring in the eastern unit of the Maures Massif (southern France Variscan belt) were emplaced in the waning stage of the evolution of a dextral shear zone along the Gondwana border with Laurasia. We present a geochronological and petro-geochemical study (major and trace elements, Sr–Nd isotopes) of (1) the foliated Moulin Blanc cordierite granite and related foliated dykes; (2) the equant Camarat granite and dyke; and (3) the andesite–basalt dolerite dykes (frequently mingled with leucogranite material at Pinet and Capon tips) and cross-cutting leucogranitic dykes. Zircon and monazite dating of the Moulin Blanc granite gives a crystallization age of 301±2Ma. SHRIMP dating on zircons from the leucogranite component of a Pinet composite dyke only reveals zircon core ages of mainly 310±10Ma (age of migmatization of the host gneiss). Undulating contacts of dolerite dykes within the c. 300Ma old Camarat granite suggest penecontemporary relationships between the two intrusions. The age interval between the Moulin Blanc granite and the dolerite dykes would thus be very short. The Moulin Blanc pluton is formed by mingling and mixing of melts of granitic and dioritic composition, the latter occurring mainly as microgranular mafic enclaves. Two other types of diorites are also identified in the pluton. The foliated dykes are similar in composition to the Moulin Blanc granite, which confirms their relationship to the main pluton. The Moulin Blanc granite is a typical example of a pluton formed by repeated recharges coming from a deeper magma chamber. Formation of the Camarat dykes is controlled by hydrothermal fluids that are responsible for the non-CHARAC behavior of trace elements. The Pinet and Capon leucogranite components of the dolerite dykes have distinct geochemical signatures, different from the Camarat granite and dykes. The cross-cutting leucogranitic dykes have much geochemical similarities with the Capon granite. The age of similar dykes in the Tanneron Massif supports the contemporaneity of Camarat granite and dolerites. The dolerite geochemistry was deeply modified by alteration processes, but an andesite–basalt composition is still recognizable by using immobile element ratios. In a εNd vs. Sri diagram, all rocks plot close to a mixing hyperbola between a depleted mantle component and a crustal component, characteristic of the Variscan crust in the French Massif Central. There is a broad correlation between the NdTDM 2-stage model ages and the degree of fractionation of the rocks. Modeling of dehydration melting in a range of P–T conditions reveals that a variety of lithologies were the source of the Saint-Tropez magmas: high-alumina and alkali-basalt amphibolites, metagraywackes, metapelites and meta-arkoses. These sources were melted almost simultaneously at different levels in the crust. The calculated critical dyke width required to transport the granitic melt by buoyancy is far too large compared to the observed granitic dyke thicknesses. Therefore, tectonic processes and/or melt overpressure must have played a role in the emplacement mechanism. Thermo-mechanical constraints on mixing between mafic and felsic melts suggest that, in the Pinet composite dykes, both melts should have mixed. The observed lack of mixing is interpreted as due to quick cooling. The region was affected by a hot zone at the end of a transpressional regime.

Mantle-derived arc related mafic enclaves and host orthogneiss from the Shyok Suture Zone of NE Ladakh, India: An evidence of magma-mixing

The Shyok valley in the northern Ladakh is a linear NW-SE trending suture zone (Shyok Suture Zone) and separates two distinct terranes, the southern continental margin of Eurasian Plate to north, and Ladakh magmatic arc to south. The crustal growth in the Shyok Suture Zone (SSZ) can be explained by two stage melting process involving the generation of orthogneiss (OG) with mafic enclaves (ME) (pre-collision), and temporal evolution towards increasingly more evolved felsic magmas during reworking processes (post-collision). In this paper, the geochemistry of various ME and host OG that fall within the Shyok‐Darbuk corridor of the suture zone are discussed. The field, petrographic and geochemical data suggest that the ME and host OG are genetically related. The ME has formed by low degree of partial melting of a garnet-bearing source or from the partial melting of a basaltic source with a clinopyroxene‐garnet eclogite as residue. The LILE and LREE enrichment of ME have been influenced by mantle‐magma interaction during the passage of the hydrous primary arc-magma through the overlying lithosphere. The likely interpretation for the genetic linkage of ME and host OG is explained through the mixing of coeval mafic (enclave) and felsic melts, resulting in hybrid magma. Besides, several geochemical lines of evidence suggest that fractional crystallization of the resultant hybrid magma had played a complementary role in the evolution of these rocks.

Kabbaldurga charnockites revisited: Incipient growth or anatectic melt?

A popular hypothesis of in situ transformation of amphibolite facies gneisses to patchy charnockites by CO2 influx from mantle was proposed primarily from the Kabbaldurga quarries in South Karnataka and subsequently reported from several south Indian localities. However, presence of abundant mafic granulite enclaves in Kabbaldurga and its neighborhood and its implications in relation to patchy charnockite genesis were not discussed. In these quarries patchy charnockites occur in various modes and associations. Some of these patches do occupy structural weak zones, such as shear bands and fold noses in the migmatitic gneisses, but many of the patchy charnockite bodies occur as branching veins transecting the gneissic foliation and hence do not account for fluid pathways. Most importantly, charnockitic leucosomes at margins of mafic granulite enclaves and charnockitic veins within some mafic granulite enclaves indicate a close genetic link between them via dehydration partial melting. This is further corroborated by trace element distribution between them. Dehydration partial melting in mafic rocks in a migmatite terrain such as Kabbaldurga, can explain all the different modes of the patchy charnockites as various stages of segregation and mobility relative to deformation. Abundant mafic granulite enclaves and field features suggesting a relatively late origin of the patchy charnockites, are compelling evidence against the notion of a transition zone. Mantle derivation age of the mafic source rocks (protoliths of mafic granulites) at Kabbaldurga at 3.08 ± 0.08 Ga with small positive ? values is virtually identical to the source of the massive charnockite of Karnataka craton at 3.08 Ga. This could imply a widespread mafic magmatism in South India around 3.0 Ga.

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.

Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith, southern California

Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and typically represent fragments of mafic magmas, cumulates, restites, or country rocks. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with the host tonalite, the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have geochemical characteristics that overlap those of literature-compiled igneous dark enclaves. When compared with modelled igneous differentiation paths, metasedimentary enclaves can have anomalous CaO and K2O contents for a given SiO2, but other major-element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that literature-compiled mafic enclaves trend towards high K2O + CaO suggests that not all mafic enclaves are of igneous origin. In this work, we provide criteria for identifying enclaves of possible metasedimentary origin.