Magmatic Enclaves Research Articles

Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Archean cratons provide windows to the crustal evolution history in the early Earth. The Dharwar Craton is one of the major cratonic blocks in Peninsular India and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3.8 to 2.5 Ga. Here we investigate a suite of magmatic rocks (including granitoids, mafic magmatic enclaves, ultramafic inclusions and synplutonic mafic dyke) from the western margin of the Eastern Dharwar Craton which provide insights on bimodal magmatism in a subduction-related arc setting and crustal growth during Neoarchean. Field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas. Geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source. We present zircon UPb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207Pb/206Pb age at 2531 Ma (n = 159) suggesting a major Neoarchean bimodal magmatic pulse. The zircon UPb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas. The trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization. Zircons from most of the rocks in our study display uniformly positive εHf(t) (0.3–8.6), suggesting depleted mantle (juvenile) source for the Neoarchean arc building, and formation of new crust. Some of the mafic magmatic and ultramafic enclaves, as well as the mafic dyke and charnockite display mixed positive and negative εHf(t) values (−7.0 to 3.8), implying mixed depleted mantle and reworked crustal components of Neoarchean to Mesoarchean age. Most of the Hf depleted mantle model ages (TDM; 2520–3115 Ma) and crustal residence ages (TDMC; 2507 Ma to 3452 Ma) fall within the region bounding CHUR and depleted mantle lines, suggesting that the dominant magma source involved juvenile (depleted mantle) components. Thus, latest Neoarchean marks a major phase of continent building in the Eastern Dharwar Craton, and the results presented in our study suggest that bimodal magmatism in a subduction-related setting identical to those in modern convergent margins contributed to crustal growth.

Geochemical and isotopic constraints on the evolution of magma plumbing system at Damavand Volcano, N Iran

Damavand is a Quaternary stratovolcano in N Iran, associated with small volumes of under-saturated to weakly-saturated mafic lavas (Tephrite-Basanite-Alkali olivine basalt), and large volumes of saturated lavas (trachyandesite-trachyte (TT)) together with minor pyroclastic rocks. TT rocks are characterized by LILE and HFSE enrichments, whereas the mafic rocks have higher REE and other incompatible elements (except Rb, Th) than TT suite rocks, contradicting the assumed derivation of TT magmas from mafic magmas by simple fractional crystallization processes. The similar whole-rock isotope ratios of mafic rocks, TT suites and their enclaves do suggest a common origin. Magmatic enclaves in TT suites show decoupled major and some trace elements from their host while they have similar geochemistry to the mafic lavas. Trace element modeling including assimilation-fractional crystallization (AFC) and energy-constrained recharge-assimilation fractional crystallization (EC-RAFC) suggest that the Damavand TT rocks were formed by multi-stage fractionation of mafic magma with minor effects of open system processes. Differentiation started at high pressures (6–8 kbar, initial temperature of magma (Tm) = 1180 °C) involving anhydrous minerals (mainly clinopyroxene-orthopyroxene), accompanied by recharge and wall-rock assimilation at equilibration temperature (Teq) of 1050 °C as the magma reached the threshold for extraction from crystal mush (Mm = 0.49) (stage-1). These processes resulted in the enrichment of the residual liquid in incompatible elements. The fractionation of oligoclase-andesine, anorthoclase, phlogopite and apatite from intermediate enriched melts occurred at shallow crustal levels (0.5–3 kbar, Tm = 1050 and Teq = 830 °C): here, FC processes and further recharge and assimilation of crustal materials reduced LILE and REE concentration in the residual liquid (stage-2). The role of enriched melts is supported by unusual trace element composition of key minerals, for instance, feldspars have variable concentrations of Ba (132–5495 ppm), Sr (1471-14413 ppm) and unusual Rb, La and Ce contents. Based upon melt extraction modeling, physical removal of residual melts at stage-2 could produce cumulate residues with trace element compositions comparable to enclaves in TT suites (stage-3). Disruption of crystal mush due to recharge, followed by mixing of enclave material with evolved trachytic melts caused buffering of magma at trachyandesitic composition (stage-4). The results affirm the advantages of using a range of compositional and textural records in modeling of petrogenetic processes in all magmatic plumbing systems.

Phase equilibria constraints on crystallization differentiation: insights into the petrogenesis of the normally zoned Buddusò Pluton in north-central Sardinia

Abstract The Buddusò Pluton in NE Sardinia (Italy) is a normally zoned intrusion composed of three units with chemical composition ranging from hornblende-bearing tonalites (SiO 2 ∼ 65 wt%) to leucocratic monzogranites (SiO 2 ∼ 76 wt%). Zircon crystals in the pluton are dated at 292.2 ± 0.7 Ma and have ε Hf values ranging from −4 to −8, with no systematic differences observed between the units. The pluton, which is isotopically homogeneous at the whole-rock scale in terms of Sr and Nd isotopes, shows textural evidence indicating local crystal–melt segregation. In this paper, we have implemented a novel approach based on path-dependent phase-equilibria modelling to test the hypothesis that the internal chemical variability of the pluton was generated by crystallization differentiation of a homogeneous parental magma. Our modelling indicates that this hypothesis is valid if the mechanism by which this occurs is compaction in a rheologically locked crystal-rich magma and if the separation occurs at 0.3 GPa from a tonalitic magma with water content >2 wt%. Finally, a subset of the magmatic enclaves in the pluton are considered to be autoliths, formed by the disruption of the compacted crystal mush and interaction between these cumulates and the felsic melt.

From magma to mush to lava: Crystal history of voluminous felsic lavas in the Gawler Range Volcanics, South Australia

The Gawler Range Volcanics (GRV) of South Australia are important extrusive analogues to the Mesoproterozoic Hiltaba Suite intrusive rocks of the Gawler Craton, which both host and are implicated in the formation of iron oxide-copper-gold deposits in the Olympic Province. We study three voluminous lavas in the GRV that were emplaced in less than one Ma and contain abundant glomerocrysts of pyroxene, titanomagnetite, plagioclase, K-feldspar, fayalite, apatite, and zircon. Within individual glomerocrysts, pigeonite can exhibit compositional zonation and a range of Y and heavy rare earth element compositions. Compositions of some pigeonite cores from within the same glomerocrysts vary, and crystal zonation does not have a systematic orientation with respect to the crystal's position within its host cluster. This suggests some phenocrysts did not grow adjacent to the other crystals found in individual clusters in the solidified rock, but rather aggregated or settled with other crystals following a stage of growth as independent, melt-bound phenocrysts. The aggregation of phenocrysts is also consistent with the multi-phase nature and diversity of grain-sizes present in the GRV glomerocrysts, rather than accumulations of crystals formed through winnowing of low-density phases or other mechanical sorting processes. In this scenario, glomerocrysts were formed partly by flocculation of independently nucleated crystals, and intergrew after aggregation during magma flow and/or as magma crystallinity increased in the magma reservoir prior to the emplacement of the lavas and the eruption trigger events. The last-erupted lava contains the highest proportion of glomerocrysts comprising complex and anhedral intergrowths. This is consistent with a normally zoned reservoir where the crystal:melt ratio and crystal concentration is greatest at depth. Loosely packed frameworks and free crystals are interpreted to occur in the middle and upper sections of the reservoir, respectively, represented by the first erupted Upper GRV lava and quartz-phyric portions of the lavas. Plagioclase is the most abundant phenocryst throughout the lavas but is present in only a subset of glomerocrysts. The low abundance and absence of feldspar and quartz in glomerocrysts is likely due to mineral resorption and subsequent separation and loss of these components from other glomerocrysts components prior to and during the eruption of the studied lavas. Once solid-solid crystal contacts were dissolved, the separation of the cluster components was driven by the stress placed on them by melt flow. Mafic magmatic enclaves recognised throughout the Upper GRV lavas likely represent the replenishing mafic magmas that provided the thermal and volatile input that drove crystal resorption. Continuing mafic replenishment and injection of sufficient volumes of magma into the Upper GRV reservoir may have also driven the eruption of the lavas. Prolonged fractionation and interaction with mafic magmas could have potentially conveyed or enhanced the distinctive A-type signature of these rocks, negating the requirement of a specialised precursor composition.

Sources and settings of Ediacaran post-collisional syenite-monzonite-diorite shoshonitic magmatism from southernmost Brazil

The Arroio do Silva Pluton comprises quartz-rich syenite-monzonite-diorite rocks, with subordinate lamprophyre as enclaves. Compositional and textural variations lead to the identification of three main varieties: (i) quartz monzonite to syenite, (ii) quartz monzonite to monzodiorite and (iii) quartz diorite. The transition between all rock types is gradational and marked mostly by the increase of mafic mineral phases, especially biotite, and by the K-feldspar content, which is characteristically euhedral in the quartz monzonite to syenite variety and becomes subhedral to anhedral towards diorite compositions. Geochemical features corroborate field relations and both point to magma mingling as the main differentiation process. Syenite is considered to be the felsic endmember, whilst the mafic one has dioritic composition. Different degrees of interaction between these magmas result in hybrid varieties represented by quartz monzonite to monzodiorite. A third magma type is observed as ultrapotassic lamprophyric enclaves, but its influence in the mixing processes seems to have been minor. Both mafic and felsic magmas have K2O/Na2O ratios between 1 and 2, compatible with their shoshonitic affinity, whilst ratios slightly above 2 characterize lamprophyric enclave magmas as ultrapotassic. The source of shoshonitic and ultrapotassic magmas, according to trace element and isotope evidence, is probably a phlogopite-bearing mantle which was affected by previous subduction of Paleoproterozoic or Neoproterozoic age. The variation of ultrapotassic to shoshonitic magmas is probably controlled by variation of phlogopite contents in the mantle source. UPb zircon crystallization age of 578 Ma (LA-MC-ICP-MS) determined for the ASP quartz monzonite to syenite variety is in agreement with the Neoproterozoic post-collisional period of the Dom Feliciano Belt. This age value is also in agreement with that of other syenitic associations from south Brazil, such as the Piquiri Syenite Massive, that records similar chemical signatures. Therefore, these syenitic associations are taken to represent previously metasomatized mantle sources in late post-collisional setting from the Brasiliano/Pan-African Cycle in southermnmost Brazil.

Magma mixing in Neoarchean granite from Nalgonda region, Eastern Dharwar Craton, India: Morphological, mineralogical and geochemical evidences

We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N–S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing–mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO, \(\hbox {TiO}_{2}\), \(\hbox {K}_{2}\hbox {O/Na}_{2}\)O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (D) show scattered behaviour in element–element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.

Neoarchean sanukitoids and associated rocks from the Tengzhou-Pingyi intrusive complex, North China Craton: Insights into petrogenesis and crust-mantle interactions

Quartz monzodiorites, granodiorites and monzogranites constitute the dominant lithological assemblage in the Neoarchean Tengzhou-Pingyi intrusive complex (TPIC) in the southwestern part of western Shandong Province (WSP) in the eastern part of the North China Craton (NCC), and mafic magmatic enclaves (MMEs) are widely developed only in granodiorites of the TPIC. LA–ICP–MS zircon U-Pb isotopic dating reveals that the TPIC was emplaced at ~2.54–2.52 Ga. The ~2.54 Ga monzogranites display geochemical affinities with high-K granites, and they were likely derived from the partial melting of metamorphic greywackes at lower crustal levels. The ~2.54–2.52 Ga quartz monzodiorites and granodiorites show significant geochemical similarities to the Archean sanukitoids of the western Karelian Province of Finland with higher Mg# (43–57) values, Cr (24–88 ppm), Sr (547–1154 ppm), and Ba (662–1961 ppm) contents and K2O/Na2O (0.42–1.00) ratios. Therefore, we define the quartz monzodiorites and granodiorites as Neoarchean sanukitoids. Based on their lithological assemblages and geochemical characteristics, these sanukitoids can be subdivided into sanukitoid quartz monzodiorites (SQMs) and porphyritic sanukitoid granodiorites (PSGs). The SQMs exhibit higher positive ɛHf(t2) values from +1.9 to +3.6 with TDM values of 2.70 to 2.77 Ga and are strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), implying a mantle source that was enriched by either fluids or melts. The PSGs have a wide range of ɛHf(t2) values from −3.4 to +3.8, suggesting that they were formed by crust-mantle magma mixing. This interpretation is also supported by the petrological textural and compositional disequilibrium of plagioclase phenocrysts from the PSGs. Moreover, the MMEs and their host PSGs have identical crystallization ages (~2.54 Ga), similar mineral chemistries and zircon Lu-Hf isotopic compositions, indicating that the MMEs are cumulates from the earlier host PSGs magma. Petrogenetic studies and zircon Lu-Hf isotopic features of these lithological assemblages indicate that the interval of ~2.55–2.52 Ga in the TPIC represents an important period of crustal growth and crust-mantle interactions. Considering the spatial-temporal relationships and the petrogenesis of the diverse lithological assemblages, we propose that the Neoarchean TPIC was most likely formed in an active continental margin setting.

Deciphering textural and chemical zoning of K-feldspar megacrysts from Mt. Amiata Volcano (Southern Tuscany, Italy): Insights into the petrogenesis and abnormal crystal growth

This study reports the complex textural and chemical features of K-feldspar megacrysts (up to 5 cm long) hosted in trachydacitic lava flows, lava domes, and coulées from Mt. Amiata volcano (Tuscan Magmatic Province, Central Italy). Backscattering and cathodoluminescence imaging, coupled with core to rim major and trace elements patterns, reveal complex zoning, and resorption surfaces associated with sharp chemical variations (e.g., Sr and Ba). These zoning patterns originated by disequilibrium and re-equilibration events, related to the repeated influx of mafic magmas or convective motions in the trachydacitic magma reservoir. Multiple mafic magma refilling occurences are also supported by the field record of abundant microgranular magmatic enclaves in the studied products. Our results highlight that the abnormal dimension of the studied K-feldspars originates by the interplay between petrological and kinetic processes involving: (i) extensive dissolution; (ii) heterogeneous nucleation; (iii) alternation of spasmodic growth events in disequilibrium and near-equilibrium crystallization. The repeated influx of hotter magmas and reheating can determine the thermal condition to the growth of few, large K-feldspar megacrysts. Also, the strong textural and chemical similarities observed in the K-feldspar megacrysts from Mt. Amiata volcanic rocks and Mt. Capanne monzogranite (Elba Island, Central Italy) support the hypothesis of a phenocrystic origin of intrusive K-feldspar megacrysts.

Insights into magma histories through silicate-oxide crystal clusters: Linking the Hiltaba Suite intrusive rocks to the Gawler Range Volcanics, Gawler Craton, South Australia

The Roxby Downs Granite (RDG), host to the supergiant Olympic Dam deposit, and the Gawler Range Volcanics (GRV) in South Australia, both contain clusters of touching silicate and oxide minerals. Here we report on the geochemistry and petrography of these rocks and their contained mineral clusters, before comparing them to similar occurrences elsewhere in the world to gain a better understand of their genesis and the links between the RDG (and other members of the Hiltaba Suite) and the GRV. Crystal clusters in the RDG comprise magnetite + apatite ± titanite ± biotite ± zircon which are typically enveloped in a plagioclase crystal-matrix. In the GRV, crystal clusters comprise pyroxene + titanomagnetite ± plagioclase ± apatite ± zircon, display variably glomeroporphyritic, equigranular, poikilitic, and loosely packed habits, and are enclosed by texturally varied groundmass. Of the GRV lavas studied, pyroxenes and titanomagnetite show systematic trends in major and trace element compositions conformable with the stratigraphic position of their hosts. This suggests the lavas are related through differentiation in the upper crust and represent the inversion of a normally zoned reservoir. There are no compositional contrasts between phenocrysts and crystals in GRV clusters, and Mg# of pyroxenes are consistent with those predicted from whole-rock compositions, implying that all analysed crystals (including those in clusters) were formed within, or were at least equilibrated to, the host lavas’ precursors. Rather than entire histories as isolated crystal clusters entrained by melt flow, intergrowth textures and mafic magmatic enclaves suggest that the majority of GRV crystal clusters were liberated from closely packed crystal masses through magma rejuvenation and associated crystal resorption. Consistent Fe-Ti oxide + apatite + zircon + plagioclase assemblages and textures in both the GRV and RDG suggest a shared genetic scenario, although an extended model is needed to account for the transformation of GRV-type to HS-type crystal clusters. For hot and dry magmas such as those that formed the GRV and RDG, cooling causes pyroxene + Fe-Ti oxide + plagioclase in GRV-type crystal clusters to react with each other and the melt, forming HS-type magnetite + titanite + biotite + feldspar crystal clusters. Rejuvenation events implicated in the liberation of GRV-type crystal clusters may be important drivers of magmatic-hydrothermal iron-oxide deposit formation.

Arc-related high-K magmatism in the Ceuta Peninsula (Internal Rif, Spain): discovery and consequences

AbstractWe document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated atc. 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions atc. 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pbin situanalyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.

Nyílt rendszerű magmás folyamatok: magmakeveredés, kristálycsere és kumulátum-recirkuláció nyomai a Ditrói alkáli masszívumban (Orotva, Románia)

A Tarniţa Komplexum a Keleti-Kárpátokban (Románia) felszínre bukkanó Ditrói alkáli masszívum északi területének jellemző ultramafikus és mafikus kőzetegyüttese. A komplexum Ny-i részén, az Orotva-patak és a Felső-Pietrăriei-patak összefolyásánál található mélységi magmás kőzettestben kialakított mesterséges feltárás egyedülálló betekintést enged az egykori magmatározóban zajlott, több magma között lejátszódott keveredési és elegyedési folya matokba, illetve az új magmabenyomulások során feltépett, korábban kikristályosodott magmás kőzetfragmen tumok bekeveredési folya mataiba. A feltárásban látható befogadó kőzetben (szürke, közép- és durvaszemcsés, irányított szövetű diorit) négy különböző keveredési kőzetzárvány (mafikus kőzetzárvány; földpátszemes kőzetzárvány; porfíros (földpátaggregátumos), mafikus kőzetzárvány és ultramafikus kőzetzárvány) és egy eltérő eredetű felzikus kőzetzárvány típus figyelhető meg. A keveredési kőzetzárványok modális összetételük alapján mezokrata és melanokrata dioritok, valamint piroxénhorn blenditek, a felzikus kőzetzárványok pedig hololeukokrata dioritok. A különböző magmák közötti keveredés (mingling) legfontosabb bélyege az egymással — és a befogadó kőzet ásványainak orientált elhelyezkedésével — párhuzamos, megnyúlt, finomszemcsés, lencse alakú kőzetzárványok meg jele nése. A szögletes felzikus xenolitok és ultramafikus kőzetzárványok korábbi kumulátumok recirkulációját felté telezik, amelyek a benyomuló magmával kerültek a magmatározóba. A keveredési kőzetzárványok mafikus pereme és a közelükben megfigyelhető „slírek” további fontos jellemzői a legalább két magma keveredése során kialakuló szerke ze teknek. A magmaelegyedés gyakori velejárója a kristálycsere (crystal transfer). A vizsgált ásványok számos esetben mutatnak olyan jellegzetes mikroszöveti bélyegeket, amelyek e kristálycsere-folyamathoz köthetők a benyomuló és a befogadó magma, illetve a benyomuló magma és a feltépett kumulátumok között: a különféle zónásságot mutató és különbözőzárványgazdag plagioklászok, az amfibolköpennyel rendelkező piroxének és a plagioklászokban megjelenő tűs apatit kristályok. A jelen tanulmányban bemutatott részletes petrográfiai vizsgálatok rávilágítanak a Ditrói alkáli masszívum magmatározóinak összetett és változatos fejlődéstörténetére, amelyben meghatározó szerepet játszottak a nyílt rendszerű magmás folyamatok.

Application and reliability of calcic amphibole thermobarometry as inferred from calc-alkaline products of active geothermal areas in the Andes

Igneous thermobarometry may contribute to locate the upper-crust heat source/s representing one of the main factors required to establish the potential of high enthalpy geothermal areas through models of the heat flux. To test this possibility, we applied amphibole thermobarometry to some of the youngest lavas and pyroclastics (andesite to rhyolite) of Apacheta and La Torta (Chile) and Chachimbiro (Ecuador) areas representing medium to high enthalpy geothermal systems located in two different crustal contexts along the Andean active continental margins of South America. We used the robust application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T ± 24 °C, P ± 12%, fO2 ± 0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1130 °C and 2.2 GPa. In addition, analyses of the intra-crystalline compositional and textural variations allowed us to propose a method to distinguish between chemically homogeneous and heterogeneous amphiboles, avoiding some incorrect applications of the Ridolfi and Renzulli (2012)'s equations and thus unreliable results.Thermobarometric constraints indicate a polybaric crystallization of magmas below the Chilean geothermal areas of Apacheta and La Torta. In the last 150 ka, the feeding systems of these areas mainly consisted of shallow silicic magma chambers at depths of 4–8 km (740–840 °C), giving rise to dacite domes (tortas). Amphiboles in the rhyolite lava flows, erupted earlier (Pliocene-Pleistocene) during the activity of the nearby Apacheta-Aguilucho Volcanic Complex, show a similar T-depth range (780–810 °C, 4–6 km). At deeper levels (10–20 km), the depths estimated using amphiboles from the magmatic enclaves (in the dacite domes) and from the andesite lavas are approximately aligned along the upper morphologic margins of the well geophysically constrained Altiplano-Puna Magma Body (APMB; a partially melted zone in the upper crust where the S-waves velocities are <2.9 km/s), with estimated temperatures of 850–1010 °C. Heat source contributions for the investigated Chilean geothermal areas should therefore be represented by both the shallower magma bodies and the huge APMB.The Holocene products of the Ecuadorian Chachimbiro geothermal area indicate amphibole crystallization from 7 to 17 km depth at temperature conditions in the range of 800–910 °C. The occurrence of two separated regions of magma storage at approximately 230 ± 30 MPa and 380 ± 60 MPa is discussed. They should be taken into account as heat sources for this Ecuadorian geothermal area.

Ar-Ar dating and petrogenesis of the Early Miocene Taşkapı-Mecitli (Erciş-Van) granitoid, Eastern Anatolia Collisional Zone, Turkey

The Early Miocene Taşkapı-Mecitli granitoid that is located in the northern section of the Eastern Anatolia Collision Zone has typical I-type, metaluminous and calk-alkaline characteristics. It also contains mafic microgranular / magmatic enclaves (MMEs). New Ar-Ar dating results show that the age of the Taşkapı-Mecitli granitoid is ∼23 Ma and it crystallised in the Early Miocene, in contrast to its previously known Cretaceous age. Identical crystallisation ages (∼23 Ma), similar mineral assemblages and geochemical compositions, and indistinguishable isotopic compositions of MMEs and host rocks imply that the MMEs are most consistent with a cumulate origin formed at earlier stages of the same magmatic system that produced the Taşkapı-Mecitli granitoid.MELTS modelling suggests that magma of the Taşkapı-Mecitli granitoid was the result of fractionation under a crustal pressure of 4 kbar, with a H2O content of 1.5%. EC-AFC model calculation reveals that the Taşkapı-Mecitli granitoid includes from 0.5% to 2% crustal assimilation rates. These rates indicate that crustal contamination can be negligible when compared to fractional crystallisation in the evolution of the magma beneath the Taşkapı-Mecitli granitoid. The partial melting model calculations and MORB-normalised trace element concentrations of the least evolved samples of the Taşkapı-Mecitli granitoid are consistent with those of mafic melts obtained from partial melting of interacting mantle- lower crust with a melting degree of 18%. The age (23 Ma) of the post- or syn-collisional Taşkapı-Mecitli granitoid suggests that the collision between Arabian and Eurasian plates could be before/around ∼23 Ma (Late Oligocene to Early Miocene).

Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age (~430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

Origin of dioritic magma and its contribution to porphyry Cu–Au mineralization at Pulang in the Yidun arc, eastern Tibet

The giant Pulang porphyry Cu–Au deposit in the Yidun arc, eastern Tibet, formed due to westward subduction of the Garze–Litang oceanic plate in the Late Triassic. The deposit is hosted in an intrusive complex comprising primarily coarse-grained quartz diorite and cored quartz monzonite. Here, we investigate a suite of simultaneous (216.6 ± 1.9 Ma) diorite porphyries within the complex. The diorite porphyries are geochemically similar to mafic magmatic enclaves (MME) hosted in coarse-grained quartz diorite, and both are characterized by low SiO2 (59.4–63.0 wt%) and high total alkali (Na2O + K2O = 7.0–9.2 wt%), K2O (3.5–6.4 wt%), MgO (3.2–5.5 wt%), and compatible trace element (e.g., Cr = 72–149 ppm) concentrations. They are enriched in large-ion lithophile and light rare earth elements (LILE and LREE, respectively), but depleted in high field-strength and heavy rare earth elements (HFSE and HREE, respectively), and yield variably high (La/Yb)N ratios (17–126, average 65) with weak to negligible Eu anomalies. Furthermore, they yield low (87Sr/86Sr)i ratios (0.7054–0.7067), weakly negative εNd(t) (−2.8 to −0.8) values, and variable zircon εHf(t) (−5.4 to +0.8) and δ18O (6.0‰–6.7‰) values. These geochemical features indicate that the diorite porphyry and MME formed through crustal assimilation of a magma produced during low-degree partial melting of metasomatized phlogopite-rich subcontinental lithospheric mantle. In contrast, the coarse-grained quartz diorite and quartz monzonite have relatively high concentrations of SiO2 (61.1–65.3 wt%), K2O (4.1–5.4 wt%), and total alkali (Na2O + K2O = 7.1–8.1 wt%), and low concentrations of MgO (generally <3.0 wt%) and compatible trace elements (e.g., Cr = 38–61 ppm). They yield high Sr/Y ratios (50–63) that indicate an adakitic affinity, and are enriched in LILE, depleted in HFSE, and yield lower (La/Yb)N values (13–20, average 17) than the diorite porphyry and MME. They yield low (87Sr/86Sr)i ratios (0.7046–0.7066), negative εNd(t) (−3.3 to −1.7) values, and zircon εHf(t) and δ18O values of −2.9 to −0.1 and 5.7‰–6.5‰, respectively, suggesting that they represent high-K calc-alkaline to shoshonitic adakitic magmas that were sourced from subduction-modified juvenile lower crust. Observations of the newly identified diorite porphyry and previously reported MME suggest that input of such dioritic magma into the upper crustal porphyry magma chamber would have contributed not only the necessary metals (e.g., Cu and Au), sulfur, but also H2O to the system, thus aiding in the generation of the giant Pulang porphyry Cu–Au deposit.

Semi-adakitic magmatism of the Satkatbong diorite, South Korea: Geochemical implications for post-adakitic magmatism in southeastern Eurasia

The Satkatbong diorite (190 Ma) and the older Yeongdeok granite (250 Ma) in the Yeongnam massif, which is part of the southeastern margin of the Eurasian plate, are affected by a subduction system that is associated with the Izanagi and Farallon plates. The Satkatbong diorite is characterized by its abundant mafic magmatic enclaves (MMEs), mantle affinity, and intermediate adakitic Sr/Y vs. Y signature, whereas the Yeongdeok granite is distinctly adakitic and felsic and contains few MMEs. These differences in adakitic features might be due to differences in the lithospheric mantle material and/or different mafic MME sources. The results of rare earth element (REE) analyses and newly proposed Sr/La modeling in this study indicate that these two plutons were both generated by slab-mantle mixing and continental assimilation, whereas the Satkatbong diorite was additionally affected by the injection of a mafic source of MMEs, which “diluted” its adakitic chemistry. The young and hot subducting ridge passing toward the northeast due to the oblique subduction of the Izanagi and Farallon plates during the Early Mesozoic could have given rise to slab melting and asthenospheric influence through slab melting regions and a slab window, respectively. This implies that the adakitic Yeongdeok granite produced by slab melting and then the semi-adakitic Satkatbong diorite produced by asthenospheric influence, including other similar adakitic to semi-adakitic magmatism, might have occurred along the areas affected by ridge subduction. We suggest that this sequential magmatism would be applicable for many continental arcs which experienced ridge subduction being one of the mechanisms of adakite to semi-adakite magmatism.

Mineralogy and mineral chemistry of andesites in comparison with their magmatic enclaves from the Ghikhlar area (East Azarbaidjan)

Mineralogy and mineral chemistry of andesites in comparison with their magmatic enclaves from the Ghikhlar area (East Azarbaidjan)

Geochemical, Sr-Nd isotopic investigations and U-Pb zircon chronology of the Takht granodiorite, west Iran: Evidence for post-collisional magmatism in the northern part of the Urumieh-Dokhtar magmatic assemblage

Subduction of Neo-Tethys lithosphere beneath the Iranian plateau during Neogene led to the formation of a NW-SE trending volcano-plutonic zone called Urumieh-Dokhtar magmatic assemblage (UDMA). The Takht granodiorite (NE of Hamedan Province, western Iran) belongs to the UDMA and has geochemical properties of post-collisional granitoids that was formed after the collision of Arabian and Iranian plateaus. This body contains rounded mafic micro-granular enclaves with relatively gradational rims indicating the effect of magma mixing/mingling in formation of the granodiorite body. The determination of U-Pb zircon age proved the Takht granodiorite was formed at Miocene (16.8 ± 0.24 Ma). The Nd-Sr isotope ratios and Sr/Nd, Nb/La and Th/U ratios of the granodiorite confirmed the magma was formed mainly by melting of continental crust, and its enclaves originated from a mantle derived mafic magma. Samples show negative anomalies in Nb, Sr, Ti, P and Eu, whereas positive anomalies in Th, K, Zr, Yb and Rb that reveals contribution of mantle and crustal materials in their generation. The Takht granodiorite has geochemical features of A2-type granites and also shows properties of both the volcanic arc and within plate magmatism association granitoids (high levels of LILEs and HFSEs). Regarding this interpretation and also post-collisional tectonic regime, it can be concluded that post-collision extensions caused deep faults in the UDMA that let mantle derived magmas rise up to the thicken crust. Such magma triggered melting in the middle crustal levels and was contaminated with crustal materials to generate granodiorite and enclave magmas respectively. The results of the current study decipher collision between the Arabian and the Iranian plateaus occurred before Miocene and the magmatism in the UDMA continued after closure of Neo-Tethys.

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.

Magmatic sulphides in Quaternary Ecuadorian arc magmas

New petrographic and geochemical data on magmatic sulphide inclusions (MSIs) are presented and discussed for 15 Quaternary volcanic centers of the Ecuadorian frontal, main and back volcanic arc. MSIs occur mostly in Fe–Ti oxides (magnetite and/or magnetite–ilmenite pair) and to a lesser extent in silicate minerals (amphibole, plagioclase, and pyroxene). MSIs are present in all volcanic centers ranging in composition from basalt to dacite (SiO2=50–67wt.%), indicating that sulphide saturation occurs at various stages of magmatic evolution and independently from the volcano location along the volcanic arc. MSIs also occur in dioritic, gabbroic and hornblenditic magmatic enclaves of the volcanic rocks. MSIs display variable sizes (1–30μm) and shapes (globular, ellipsoidal, angular, irregular) and occur mostly as polymineralic inclusions composed of Fe-rich and Cu-poor (pyrrhotite) and Cu-rich (mostly chalcopyrite) phases. Aerial sulphide relative abundances range from 0.3 to 7ppm in volcanic host rocks and from 13 to 24ppm in magmatic enclaves. Electron microprobe analyses of MSIs indicate maximum metal contents of Cu=65.7wt.%, Fe=65.2wt.%, Ni=10.1wt.% for those hosted in the volcanic rocks and of Cu=57.7wt.%, Fe=60.9wt.%, Ni=5.1wt.%, for those hosted in magmatic enclaves. Relationships of the sulphide chemistry to the host whole rock chemistry show that with magmatic differentiation (e.g., increasing SiO2) the Cu and Ni content of sulphides decrease whereas the Fe and S contents increase. The opposite behavior is observed with the increase of Cu in the whole rock, because the latter is anti-correlated with the SiO2 whole rock content. Laser ablation ICP-MS analyses of MSIs returned maximum values of PGEs and noble metals of Pd=30ppm, Rh=8.1ppm, Ag=92.8ppm and Au=0.6ppm and Pd=43ppm, Rh=22.6ppm, Ag=89ppm and Au=1ppm for those hosted in volcanic rocks and magmatic enclaves, respectively. These PGE contents display a different range of values with respect to those in previously investigated magmatic sulphides. MSIs that are Cu- and PGE/Cu-rich are found in less evolved rocks (i.e., lower SiO2 contents) that also display a lower amount of sulphide inclusions. Cu-rich sulphide phases (chalcopyrite±bornite) are mostly hosted by magnetite, whereas PGE-rich ones consist of a Cu-poor phase (pyrrhotite) hosted by plagioclase. However, no systematic changes in the chemistry of the host silicate mineral are observed in coincidence with the occurrence of MSIs. We use the results of our study to draw some implications on Cu (and other chalcophile elements) behavior during arc magmatic processes potentially associated with the formation of porphyry-type deposits.