Magmatic Enclaves Research Articles

Temporal Changes of Magmas That Caused Lava-Dome Eruptions of Haruna Volcano in the Past 45,000 Years

We have studied four lava dome eruptions which occurred in 45–10 ka in Haruna volcano. Enclave parts (SiO2 50.9–55.1 wt%) and host parts (SiO2 59.5–64.5 wt%) in lava samples are all products of magma mixing. Characteristics of felsic endmember magmas are the same among four eruptions, while those of mafic endmember magmas vary slightly in terms of bulk composition. The felsic magma had SiO2 ≥ 63 wt% and a temperature of 760°C–860°C, and contained ≥60 vol% of orthopyroxene, amphibole, plagioclase, quartz, and Fe-Ti oxides. The mafic magma had SiO2 48–51 wt% and contained 0–10 vol% of olivine. The enclave magmas resulted from higher contribution of mafic magma and thus had higher temperature than the host magmas, which led to formation of enclave upon their interaction. Similarities of endmember magmas between the four eruptions and the Futatsudake-Ikaho eruption (late 6th–beginning of 7th century) suggest structure of magma plumbing system and eruption triggering process have been basically unchanged in past 45,000 years. The felsic magmas were commonly mush-like and had high viscosity. Therefore, generation of low-viscosity magma through magma mixing, and vent-opening by the low-viscosity magma are mandatory for eruption to initiate. Unlike the Futatsudake-Ikaho eruption, the older four eruptions did not proceed to eruptive phase where felsic magma erupts without mixing and explosively. The absence of quartz only in felsic magma of Futatsudake-Ikaho eruption is consistent with its less-evolved bulk composition and slightly higher temperature than those of older four eruptions.

Effect of source compositions on adakitic features: A case study from the Buya granite, in western Kunlun, NW China

Characterized by high Sr/Y (> 40) and La/Yb (> 20) ratios, the adakitic features are, generally, attributed to a garnet-bearing residue. However, adakitic features may be inherited from an adakitic source. In this contribution, we take the Buya granite as an example to demonstrate the effect of source on adakitic features through petrology, zircon U-Pb geochronology and Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes. The 445 Ma Buya granite contains many coeval microgranular magmatic enclaves (MMEs), and some late fine-grained granitic dikes. The host granite and the MMEs display sub-parallel to parallel trace-element patterns characterized by enrichments in LREE and LILE, and depletions in HREE and HFSE, typical of adakitc features. Importantly, the two lithologies possess comparable zircon Hf and whole rock Sr-Nd-Pb isotopic compositions that plot along a mantle array. The MMEs are interpreted as cognate fragments of early-formed crystals and the parental magma was sourced from overriding mantle wedge metasomatized by fluids derived from the subducted Proto-Tethys slab. Comparatively, the dikes, with positive εNd(t) (∼2) values and lower (87Sr/86Sr)t (∼0.705), were products of the partial melting of a plagioclase-rich crustal source. Although the host granite and the MMEs possess adakitic features, the latter have higher La/Yb ratios and relatively lower Sr/Y ratios, which are inconsistent with garnet effect because of similarly high DY/DSr (∼1900) and high DYb/DLa ratios (∼2300) for garnet/melt. Instead, this feature is attributed to the fractional crystallization of hornblende and allanite. Therefore, the host granite and the MMEs may inherit adakitic features from the source. Likewise, the adakitic features of the dikes are attributed to a plagioclase-rich source due to the enrichment in Sr element. The early fractional crystallization of amphibole and biotite in the MMEs can elevate Zr/Sm and lower Nb/Ta ratios, respectively, in the residual liquid to form evolved magma similar to adakites and TTGs. The Buya granite displays an arc affinity, demonstrating the northward subduction of the Proto-Tethys oceanic plate.

Mingling with minimal mixing: Mafic-silicic magma interactions in the Oamikaub ring complex, Namibia

The ca. 560 Ma Oamikaub ring complex (ORC) is a member of the Goas Intrusive Suite in the Damara Belt of Namibia. The volumetrically dominant granitic rocks contain a variety of microgranular magmatic enclaves (MEs), commonly with lobate and cuspate borders against their enclosing host rocks. Some also have mafic selvages and evidence for internal melt segregation. These features indicate magma mingling and deformation during host-magma flow and emplacement, and a small degree of chemical mixing with the host magmas. The spatial diversity and mineralogical characteristics of the MEs show that mechanical hybridism occurred prior to mingling at emplacement level. Despite one group of MEs forming near-linear arrays in chemical plots, the mineralogical and isotopic data show that these do not form a mixing series. A second group represents samples of chilled mafic magmas that chemically resemble the mafic rocks that form the earliest-emplaced components of the ORC. However, their fine-grained textures and isotopic characteristics show that these were a separate, later pulse of mafic magmas. There were multiple episodes of mingling and the younger pulses of mafic ORC magmas were generally more hybridised with evolved crust. Magma mingling and limited marginal hybridisation of MEs produced impressive outcrop features. However, apart from initial, source-level hybrid formation, magma mixing had little or no role in producing compositional variations in either the MEs or their host rocks. Globally, such relationships are common in enclave suites from granitic rocks, and their study can provide insights into processes and materials in magma source regions. In the present case, enriched mantle melting, crustal melting (of mafic to silicic rocks) and fractionation of tholeiitic parental magmas may all have played their parts in forming the ME suite of the ORC.

Crustal maturation and cratonization in response to Neoarchean continental collision: The Suizhong granitic belt, North China Craton

The late Archean (3.0–2.5 Ga) is a pivotal period, when properties of the continental crust significantly changed and a fundamental shift of geodynamic processes took place. As the major felsic component in early Archean (> 3.0 Ga) crustal records, tonalite-trondhjemite-granodiorite (TTG) suites are compositionally different to the present-day felsic upper continental crust. But it remains unclear how and to what degree the felsic part of the continental crust evolved during the late Archean. The 250 km long Neoarchean Suizhong granitic belt in the North China Craton (NCC) is mainly composed of K-rich granitoids and compositionally distinct to the TTG-dominated early Archean upper continental crust. This belt is a natural laboratory to explore how the early Archean upper continental crust evolved in the late Archean. Combined with our new data (zircon U-Pb ages, bulk-rock geochemistry and zircon Hf isotopes), we provide an overview of the temporal and compositional features of the Neoarchean Suizhong granitic belt. Granitic magmatism in the Suizhong granitic belt mainly took place within 100 Myrs around the end of the Archean, and these rocks are primarily K-rich granitoids with minor TTGs and mafic magmatic enclaves (MMEs). The K-rich granitoids include sanukitoid-like granites and potassic granites; the former are hybrids between melts from Mesoarchean enriched mafic crust and enriched mantle, and the latter are melts from Hadean–Mesoarchean crustal lithologies. The volumetrically minor TTGs were sourced from Mesoarchean low-K mafic crust. MMEs are either cumulates or inclusions of captured enriched mantle melts. Overall, these Neoarchean granitoids are essentially intracrustal reworking products of ancient continental nuclei and are compositionally and temporally similar to syn- and post-collisional magmatism in Phanerozoic collisional orogenic belts. The Neoarchean crustal reworking process was most likely to be induced by amalgamation of micro-continents through collision. Through the collision-induced reworking, a variety of K-rich granitoids were introduced to the Neoarchean upper continental crust. As a result, layering of felsic upper and mafic lower crust was developed, and the maturity of Archean continental crust was significantly enhanced. Collisional orogenesis played a critical role in maturing the Archean continental crust and initial cratonization of the NCC during the late Archean.

Simultaneous determination of zircon crystallisation age and temperature: Common thermal evolution of mafic magmatic enclaves and host granites in the Kurobegawa granite, central Japan

• Zircon U-Pb age and Ti concentration are simultaneously determined by LA-ICP-MS. • This method aids in determining the t–T paths of a magma chamber . • This method allows us to directly relate zircon crystallisation age to temperature. • MMEs and host rocks in the Kurogebawa granite show common cooling behaviours. • This method better defines magma chamber processes and formation history of MMEs. Simultaneous determination of the U-Pb age of zircon and concentration of titanium in a single analysis spot, using inductively coupled plasma mass spectrometry with laser ablation sample introduction, produces paired age and temperature data of zircon crystallisation, potentially revealing time–temperature ( t–T ) histories for evolved magma. The Kurobegawa granite, central Japan, contains abundant mafic magmatic enclaves (MMEs). We applied this method to evaluate MMEs and their host (enclosing) granites. Cooling behaviour common to both MMEs and host rocks was found between 1.5 and 0.5 Ma. Rapid cooling from the zircon crystallisation temperature to the closure temperature of biotite K-Ar system was within ∼1 million year. Combining the obtained t–T paths of MMEs and host rocks with petrological information can provide insights into magma chamber processes. This suggests that MME flotation, migration, and spread through the magma chamber ceased at 1.5–0.5 Ma, indicating the emplacement age of the Kurobegawa granitic pluton, as no large-scale reheating episodes have occurred since then.

Permian Remelting and Maturity of Continental Crust Revealed by the Daqing Peraluminous Granitic Batholith, Inner Mongolia

AbstractThe Phanerozoic granites in northeast China bear key information for studying the tectonic evolution and crustal growth or reworking in the Central Asian Orogenic Belt (CAOB). The Daqing granitic batholith widely outcrops as a high-level intrusion in the Xing’an-Mongolia Orogenic Belt, southeastern CAOB. Three types of enclaves in granites have been identified: (1) mafic magmatic enclaves (MMEs), (2) volcanic xenoliths, and (3) biotite-rich enclaves. The batholith is mainly composed of peraluminous biotite granite and granodiorite with SiO2=63.95−69.48 wt.%, A/CNK=1.15−1.27, and 2.54 to 4.30 wt.% of normative corundum. They exhibit remarkable enrichment in large ion lithophile elements (LILEs; K, Rb, Th, and Pb) and depletion in high field strength elements (HFSEs; Nb, Ta, and Ti), P, Eu, and Sr, as well as relatively enriched Sr-Nd isotopes (87Sr/86Sri=0.70530−0.70576, εNdt=–0.1−+0.2). Zircon U-Pb dating suggests that this batholith was emplaced in the Early Permian (ca. 283-282 Ma), consistent with a period of intensive magmatic activities in northern Inner Mongolia. The Nb/Ta ratios of MMEs (17.6-20.1) are higher than those of the host granites (11.4-12.5), together with the reaction rims where biotite crystals cluster around the amphibole cores, suggesting magma mixing between mantle- and crust-derived melts. Zircons from a biotite-rich enclave define a protolith age of ca. 320 Ma and an anatectic age at ca. 281 Ma. Whole-rock Sr-Nd isotopic modelling and zircon Hf isotopes reveal that the batholith was mainly produced by remelting of newly accreted continental crust with minor addition of mantle-derived materials. The geochemical compositions imply that their precursor magmas originated from a relatively high crustal level (<5 kbar) with crystallization temperatures ranging from 800 to 850°C. We suggest that the Daqing peraluminous granitoids were derived from partial melting of newly accreted crustal materials at a relatively shallow crustal depth, associated with a ridge subduction-related heat source. Such mantle-derived magmas through a slab tear window resulting from ridge subduction provide not only the heat for the widespread crustal remelting and therefore maturity but also juvenile materials for crustal growth.

Re-assessment of the effect of fractional crystallization on Mo isotopes: Constraints from I-type granitoids and their enclosed mafic magmatic enclaves

In recent years, considerable progress has been made on the study of Mo isotopes in high-temperature geological processes. However, it is still controversial whether Mo isotope fractionation occurs during magmatic differentiation. Here we reassess the effect of magma differentiation on Mo isotope fractionation using published data in conjunction with our new analysis of Mo isotopes in well-characterized I-type granitoids and their enclosed mafic magmatic enclaves (MMEs) from two plutons in the North Qilian Orogen. The MMEs in each pluton, representing earlier cumulates with greater modal proportions of amphibole and biotite, show similar Mo elemental and isotopic compositions to their host granitoids. The absence of covariation of Mo isotopes with Dy/Dy*, Fe2O3 and K/Rb in both plutons further indicates that fractional crystallization of amphibole, biotite and Fe3+-rich minerals does not fractionate Mo isotopes in these granitoids. On the basis of re-assessment of published and our newly obtained Mo isotope data, we find that there is no clear evidence for Mo isotope fractionation during fractional crystallization in igneous rock. Instead, Mo isotopes of our samples correlate positively with Sr isotopes and Rb/La ratios, and negatively with Nd isotopes and Ce/Pb ratios, indicating clearly that magma source compositional variation controls the variation of Mo isotope compositions of these granitoids.

Magmatic Processes at Euganean Hills (Veneto Volcanic Province, Italy): Clinopyroxene Investigation to Unravel Magmatic Interactions

The Euganean Hills (NE Italy) magmatic district represents the final volcanic activity of the Veneto Volcanic Province. Alkaline to subalkaline magmatic suite dominated by intermediate to felsic volcanic rocks characterises the latest volcanic activity of the Euganean Hills. Magmatic (intrusive and volcanic) enclaves are common in Euganean Hills trachytes. We used the ability of clinopyroxene to record variations of P, T, and fO2 to reconstruct the geological history of the volcanic enclaves and trachytic host. Despite similar major and trace elements composition, clinopyroxene from host is higher in Ca and Na (and Fe3+) and lower in Mg than enclaves and is slightly enriched in trace elements but with the same pattern distribution. Minor differences in geochemistry and crystal structure of clinopyroxene from enclaves and trachytic host suggest similar parental magmas that differs by small degrees of fractional crystallisation. Clinopyroxene geobarometry performed combining X-ray diffraction with mineral geochemistry for volcanic enclaves–trachytic host combined with amphibole geobarometry for intrusive enclaves and crystal mushes points to a crystallisation pressure range between 4.8–2.0 kbars. Our data support the model of a complex system of magma chambers at intermediate to shallow crustal level where mafic magma accumulated, evolved by fractionation processes and mixed.

Late Neoarchean Crust-Mantle Interaction and Tectonic Implications in the Western Shandong Province: Evidence from a Granodiorite Pluton and Associated Magmatic Enclaves

Late Neoarchean Crust-Mantle Interaction and Tectonic Implications in the Western Shandong Province: Evidence from a Granodiorite Pluton and Associated Magmatic Enclaves

FIELD AND PETROGRAPHIC EVIDENCES OF GRANITOIDS AND MAFIC MAGMAS INTERACTION IN THE NEOPROTEROZOIC CENTRAL AFRICAN FOLD BELT IN CAMEROON (MAKENENE AREA)

Field and petrographic studies of the Makenene area in the Central African Fold Belt in Cameroon reveals several features testifying the mafic and felsic magmas interactions and their coeval nature. They are: (1) the Mafic Magmatic Enclaves (MMEs) scattered throughout the Makenene granitoid pluton and displaying sub-rounded shape and back veining, (2) flow structures consisting of schlierens at the tails of MMEs, folded MMEs along with felsic host granitoids with hinge indicating the flow direction, (3) irregular or cuspate boundary between MMEs and host granitoids, (4) quenching of apatite and biotites minerals, (5) MMEs enclosing other MMEs or felsic host granitoids. The mafic magma injection operated during at least four stages (from early to late crystallization state of the host magma) leading respectively to the formation of homogenized granitoid; sub-spherical MMEs scattered in the pluton; dismembered dyke and undisturbed synplutonic mafic dyke. The Makenene area registered four deformation phases (D1 to D4). The first two deformation phases occurred before the magmatism and migmatization events. The third phase is coeval to the magmatism and the migmatization period (at the Eburnean orogeny (2.08-2.07 Ga)). The fourth phase is related to the Panafrican orogeny.

Middle-Late Jurassic magmatism in the west central Lhasa subterrane, Tibet: Petrology, zircon chronology, elemental and Sr-Nd-Pb-Hf-Mg isotopic geochemistry

Mesozoic magmatic rocks are widespread in the Lhasa terrane, but most of them are of cretaceous age. Because Jurassic rocks are relatively rare and our knowledge on such earlier magmatism in the context of the tectonic setting and evolution is limited. In this study, we focus on the mid-late Jurassic granitoids that occur in the west central Lhasa subterrane. We present the results of a systematic study of these granitoid rocks of tonalite composition together with the hosted mafic magmatic enclaves. We dated 4 representative tonalite samples and 2 enclaves using zircon U-Pb method that gives the age range of 167–154 Ma. All these samples have Sr-Nd-Pb isotopic compositions (87Sr/86Sr = 0.713941–0.718417, εNd(t) = −14 to −9.8, 206Pb/204Pb = 18.806–18.936, 207Pb/204Pb = 15.739–15.764, 208Pb/204Pb = 39.257–39.798) similar to the composition of gneisses from the basement of the Lhasa terrane, suggesting that magmas parental to these mid-late Jurassic granitoids of tonalitic composition are of largely crustal origin, which is also supported by the petrographic observations. Both of the tonalite samples and the mafic enclaves have a wide span of zircon εHf(t) of −15.9 to −0.2 and − 13.8 to −7.4, respectively. These samples also show varying δ26Mg ranging from −0.40 to −0.18, with an average δ26Mg = −0.27 ± 0.06‰ (2SD) that is best understood as representing the Mg composition of the continental crust of the Lhasa terrane. All these observations allow us to conclude that basaltic melts derived from metasomatized mantle were involved in the petrogenesis of these granitoids. Specifically, southward subduction of the Bangong-Nujiang Ocean lithosphere and subsequent slab rollback metasomatized the mantle wedge and the lithospheric mantle above, whose melting produced basaltic magmas. Underplating and intrusion of these basaltic magmas caused crustal melting and generation of granitoid magmas parental to the tonalite and enclaves we study.

Identifying Crystal Accumulation in Granitoids through Amphibole Composition and In Situ Zircon O Isotopes in North Qilian Orogen

Abstract Granitoids are the main constituents of the continental crust, and an understanding of their petrogenesis is key to the origin and evolution of continents. Whether crystal fractionation is the dominant way to generate evolved magmas has long been debated, mostly because such processes would produce large volumes of complementary cumulates, which remains elusive. Mafic magmatic enclaves (MMEs) are ubiquitous in granitoids and their presence was initially recognized as cumulates. However, because many MMEs lack obvious evidence of accumulation, such as the classic cumulate textures and modal layering, the cumulate origin of MMEs has been abandoned and the model of magma mixing between mafic and felsic magmas has become popular. In this study, we conduct a combined study of amphibole composition and in situ O isotopes in zircons on three suites of orogenic granitoids with MMEs from the North Qilian Orogenic Belt (NQOB). We find that the MMEs and their host granodiorites show overlapping zircon δ18O values, affirming that they share the same parental magmas. The amphibole compositions indicate that amphiboles from the MMEs are not in equilibrium with a melt whose composition was that of the bulk-rock. These new data, together with the published bulk-rock data, suggest that the MMEs in our study have clear cumulate signatures and are thus of cumulate origin. Our study provides evidence for crystal accumulation in granitoids in the NQOB. This new understanding calls for re-examination on the petrogenesis of some intermediate magmatic rocks (granitoid/andesite) in discussing models of continental crustal growth.

Syn-kinematic emplacement of granitic batholith and leucogranite along the extensional detachment shear zone system of the Tin Begane area, Laouni terrane (LATEA metacraton, Central Hoggar, Algeria)

The vast Tin Begane batholith (5000 km2) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 ± 3 Ma, U–Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 ± 4, U–Pb zircon) and a fine-grained monzogranite (591 ± 6 Ma, U–Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 ± 6 Ma, U–Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleoproterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 °C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 °C) not far from the emplacement of the Tin Begane batholith.A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.

Mesoscopic and Microscopic Magmatic Structures in the Quxu Batholith of the Gangdese Belt, Southern Tibet: Implications for Multiple Hybridization Processes

The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.

Increasing sulfur and chlorine contents in ore-forming magmas: The key to Pulang porphyry Cu-Au formation, SW China

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/t Au) is located in the southern Yidun terrane of the Sanjiang (Three-river) region, southwest China. It has six phases of porphyry intrusions: premineralized fine-grained quartz diorite (FQD) and coarse-grained quartz diorite porphyry (CQD), the main synmineralized quartz monzonite porphyry (QMP), and late synmineralized granodiorite porphyry (GP), diorite porphyry (DP), and aplite. New zircon U-Pb ages for the premineralized FQD and late synmineralized GP, DP, and aplite are 226.3 ± 2.6 Ma (MSWD = 1.9), 216.4 ± 1.0 Ma (MSWD = 2.0), 216.0 ± 1.1 Ma (MSWD = 1.7), and 216.0 ± 1.5 Ma (MSWD = 1.7), respectively. All Pulang porphyry rocks have high whole-rock Sr/Y ratios (37.8–184) with minor or no negative Eu anomalies (0.73–1.04), and are characterized by high large-ion-lithophile-element (LILE: Th, U, Ba, Rb) abundances and low high-field-strength-element (HFSE: Nb, Ta, Ti, Zr, Hf).Zircons from premineralized FQD and synmineralized GP, DP, and aplite rocks have consistently high EuN/EuN* ratios (mostly > 0.4). The magmatic oxygen fugacity calculated from zircon compositions, show that the FQD (ΔFMQ = 1.59 ± 0.43), GP (ΔFMQ = 1.52 ± 0.18), DP (ΔFMQ = 1.52 ± 0.30), and aplite (ΔFMQ = 1.75 ± 0.60) were all derived from moderately oxidized magmas. Magmatic water contents estimated from amphibole compositions and the extensive presence of amphibole phenocrysts suggest that H2O contents are >4 wt%. Together with the published results for premineralized CQD and main synmineralized QMP, it is concluded the causative magmas for the six porphyry phases at Pulang were all relatively oxidized and hydrous.The difference between premineralized FQD and CQD and synmineralized QMP, GP, and DP are reflected in magmatic chlorine and sulfur contents. To minimize the effect of hydrothermal alteration, the apatite microphenocrysts enclosed by least altered phenocrysts were selected for analyses. The Cl contents of magmatic apatite microphenocrysts from the synmineralized QMP (avg = 0.340 ± 0.192, n = 26), GP (avg = 0.317 ± 0.151, n = 21), and DP (avg = 0.179 ± 0.112, n = 18) are significantly higher than those from the premineralized FQD (avg = 0.046 ± 0.032, n = 9) and CQD (avg = 0.043 ± 0.034, n = 13). Similarly, the SO3 contents of igneous apatite microphenocrysts from the synmineralized QMP (avg = 0.259 ± 0.156, n = 25), GP (avg = 0.203 ± 0.163, n = 20), and DP (avg = 0.306 ± 0.173, n = 18) are higher than those from the premineralized FQD (avg = 0.181 ± 0.136, n = 10) and CQD (avg = 0.085 ± 0.068, n = 6). The S-Cl contents of synmineralized melts estimated from apatite S-Cl compositions are higher than those of premineralized magmas, suggesting that the elevated magmatic S-Cl contents in the synmineralized porphyries might have been important for Cu-Au mineralization. This was likely due to S-Cl rich mafic magma recharge, supported by the presence of abundant mafic magmatic enclaves in the synmineralized QMP.

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

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

Generation of the Giant Porphyry Cu-Au Deposit by Repeated Recharge of Mafic Magmas at Pulang in Eastern Tibet

Abstract The giant Pulang porphyry Cu-Au district (446.8 Mt at 0.52% Cu and 0.18 g/t Au) is located in the Yidun arc, eastern Tibet. The district is hosted in an intrusive complex comprising, in order of emplacement, premineralization fine-grained quartz diorite and coarse-grained quartz diorite, intermineralization quartz monzonite, and late-mineralization diorite porphyry, which were all emplaced at ca. 216 ± 2 Ma. Mafic magmatic enclaves are found in both the coarse-grained quartz diorite and quartz monzonite. The well-preserved primary mineral crystals in such a systematic magma series (including contemporaneous relatively mafic intrusions) with well-defined timing provide an excellent opportunity to investigate upper crustal magma reservoir processes, particularly to test the role of mafic magma recharge in porphyry Cu formation. Two groups of amphibole crystals, with different aluminum contents, are observed in these four rocks. Low-Al amphibole crystals (Аl2О3 = 6.2–7.6 wt %) with crystallization temperatures of ~780°C mainly occur in the coarse-grained quartz diorite and quartz monzonite, whereas high-Al amphibole crystals (Al2O3 = 8.0–13.3 wt %) with crystallization temperatures of ~900°C mainly occur in the fine-grained quartz diorite and diorite porphyry. These characteristics, together with detailed petrographic observations and mineral chemistry studies, indicate that the coarse-grained quartz diorite and quartz monzonite probably formed by crystal fractionation in the same felsic magma reservoir, whereas the fine-grained quartz diorite and diorite porphyry formed from relatively mafic magmas sourced from different magma reservoirs. The occurrence of mafic magmatic enclaves, disequilibrium phenocryst textures, and cumulate clots indicates that the coarse-grained quartz diorite and quartz monzonite evolved in an open crustal magma storage system through a combination of crystal fractionation and repeated mafic magma recharge. Mixing with incoming batches of hotter mafic magma is indicated by the appearance of abundant microtextures, such as reverse zoning (Na andesine core with Ca-rich andesine or labradorite rim overgrowth), sharp zoning (Ca-rich andesine or labradorite core with abrupt rimward anorthite decrease) and patchy core (Ca-rich andesine or labradorite and Na andesine patches) textured plagioclase, zoned amphibole, high-Al amphibole clots, skeletal biotite, and quartz ocelli (mantled quartz xenocrysts). Using available partitioning models for apatite crystals from the coarse-grained quartz diorite, quartz monzonite, and diorite porphyry, we estimated absolute magmatic S contents to be 20–100, 25–130, and >650 ppm, respectively. Estimates of absolute magmatic Cl contents for these three rocks are 1,000 ± 600, 1,800 ± 1,100, and 1,300 ± 1,000 ppm, respectively. The slight increase in both magmatic S and Cl contents from the premineralization coarse-grained quartz diorite magma to intermineralization quartz monzonite magma was probably due to repeated recharge of the relatively mafic diorite porphyry magma with higher S but similar Cl contents. Mass balance constraints on Cu, S, and Cl were used to estimate the minimum volume of magma required to form the Pulang porphyry Cu-Au deposit. Magma volume calculated using Cu mass balance constraints implies that a minimum of 21–36 km3 (median of 27 km3) of magma was required to provide the total of 2.3 Mt of Cu at Pulang. This magma volume can explain the Cl endowment of the deposit but is unlikely to supply the sulfur required. Recharge of 5–11 km3 of diorite porphyry magma to the felsic magma reservoir is adequate to account for the additional 6.5–15 Mt of S required at Pulang. Repeated diorite porphyry magma recharge may have supplied significant amounts of S and some Cl and rejuvenated the porphyry system, thus aiding formation of the large, long-lived magma reservoir that produced the porphyry Cu-Au deposit at Pulang.

Thermo-mechanical destruction of Archean cratonic roots: Insights from the Mesozoic Laiyuan granitoid complex, North China Craton

Igneous complexes with diverse magmatic suites provide essential targets for investigating magma differentiation processes and crust-mantle interaction. The North China Craton (NCC) is a crucial window to gain insights into craton destruction through voluminous magmatism during the Mesozoic involving extensive crust-mantle interaction. Here we investigate the granitoid suite from the Laiyuan complex in the central NCC to constrain the magmatic evolution and petrogenetic linkages and to gain insights into the process of crust-mantle interaction related to craton destruction. The Laiyuan granitoid complex comprises syenogranite, monzogranite, quartz monzonite and monzonite with mafic magmatic enclaves (MMEs). The granitoids were formed during the Early Cretaceous (137–128 Ma) with εHf(t) values and TDMC ages ranging from −21.8 to −16.8 and 2564 to 2255 Ma, respectively. The compositional variations of the different rock types suggest distinct magmatic evolution and petrogenesis. Geochemical and isotopic data indicate that the enriched lithospheric mantle-derived mafic magmas underwent early-stage plagioclase-dominated and late-stage hornblende-dominated fractional crystallization to form the monzonites and quartz monzonites. Geochemical modeling results suggest that the magma of the MMEs originated through mixing between lithospheric mantle-derived mafic magma and crust-derived felsic magma with post-injection hybridization to generate MMEs with variable compositions. The monzogranites are classified as high-K calc-alkaline I-type granites with higher SiO2 contents (69–71 wt%) and lower Mg# (41–44) than intermediate rocks. They were generated by partial melting of the basaltic lower crust, with the addition of mafic magma from an enriched mantle. The highly fractionated syenogranites with lower Mg# (18–27) were evolved from the monzogranitic magma through plagioclase-dominated crystal removal. The adakitic affinities of the monzonites and monzogranites were generated by crystallization of mantle-derived magmas and partial melting of continental crust of normal thickness (~40 km), respectively. The bottom-up crust-mantle interaction accounts for the compositional diversity of the Laiyuan granitoids, and suggests that thermo-mechanical destruction played a dominant role in the destruction of the central NCC as compared with the continental delamination process in the eastern NCC.

T-P-fO2 conditions of sulfide saturation in magmatic enclaves and their host lavas

Physicochemical conditions under which early sulfide saturation occurs and the role of this process in the mineralising potential of magmas in different geodynamic settings have been the subject of interest in many recent studies. Here we present new temperature-pressure-fO2 data on sulfide-saturated magmatic enclaves and host lavas from volcanic systems in subduction, post-subduction and intraplate geodynamic settings, some of which are associated with porphyry and epithermal deposits. Petrographic investigations coupled with mineral chemistry of sulfide inclusions and their host minerals and with bulk chemistry of amphibole, plagioclase and pyroxene-rich enclaves and their host lavas indicate that sulfide occurrence, abundance, and composition depend on magma evolution. Sulfides are more abundant and have higher Ni/Cu bulk ratios when found in mafic enclaves compared to sulfide inclusions occurring in more evolved lavas. Thermo-oxibarometry estimates indicate that mafic hornblende-rich cumulates (SiO2 < 45 wt%) occurring at the studied intraplate setting evolve and reach sulfide saturation already in the mantle (61 ± 7 km) at high temperature and pressure conditions (17 kbar and 1135 ± 28 °C) and low ΔNNO (< −1). These cumulates are associated with a greater sulfide abundance (up to 0.23 area %) compared to other types of enclaves and have Cu-poor sulfides (mostly pyrrhotite, with Cu median = 0.16 wt%) with high Ni/Cu values (up to 65). In contrast, more evolved (SiO2 > 50 wt%) gabbroic plutonic enclaves found in arc settings saturate sulfides at shallower crustal levels (~26 ± 10 km, i.e. ≤ 7 kbar) and at lower temperature (1014 ± 28 °C) and higher ΔNNO (> +1), and are characterised by a lower sulfide abundance (down to 0.01 area %), dominated by Cu-rich sulfides (mostly chalcopyrite and bornite with Cu median = 50 wt%). By accounting for the sulfide volume, our results suggest that sulfides can retain significantly higher amounts of metals in the enclaves (Cu = 32–113 ppm) compared to sulfides in the host lavas (Cu < 8 ppm). Considered altogether, our results indicate that alkaline mafic, less oxidised, and barren magmas corresponding to intraplate and back-arc settings saturated in sulfides in the upper mantle. In contrast, calc-alkaline-felsic, oxidised and ore-related magmas corresponding to convergent margins started to be saturated in sulfides at the expected depth of the MASH zone and at shallower crustal levels. This seems to be a consequence of the investigated alkaline systems not forming shallow crustal magma chambers allowing magma differentiation, rather than the fact that late and shallow sulfide saturation is necessary for porphyry formation.

Early Mesozoic crustal evolution in the NW segment of West Qinling, China: Evidence from diverse intermediate–felsic igneous rocks

Continental convergent margins are commonly considered as significant sites for understanding crustal reworking and growth, but the geodynamic processes involving crustal growth remain highly debatable. Triassic intermediate–felsic intrusive rocks are widespread in the northwestern segment of the West Qinling orogenic belt on the NE Tibetan Plateau. These rocks well document the evolution of one branch of the Paleo-Tethys oceans (i.e., the A'nyemaqen Ocean) in East Asia. Here we present a comprehensive study of zircon UPb and LuHf isotopic data, along with whole-rock elemental and SrNd isotopic data to decipher the origins of intermediate–felsic intrusions and crustal evolution of the West Qinling in the Triassic. Two episodes of magmatism were detected in this study, i.e., the early high-Mg diorite porphyry–granodiorite–granodiorite porphyry assemblage (Episode #1, ~244–230 Ma), followed by a later monzogranite–granodiorite–granite porphyry assemblage (Episode #2, ~228–203 Ma). These two episodes of magmatism are interpreted as products related to oceanic subduction and continental collision, respectively. In Episode #1, the mafic magmatic enclaves were mainly mantle-derived and were indicative of magma mixing. The high-Mg diorite porphyries were likely derived from partial melting of the mantle wedge metasomatized by sediment-derived melts, whereas the coeval granodiorites and granodiorite porphyries mainly originated from partial melting of amphibolites with significant addition of mantle-derived components. By contrast, the granodiorite porphyries in Episode #2 probably originated from the lower crust-sourced melts with insignificant mantle contribution. The contemporaneous granodiorites and monzogranites were generated by partial melting of the crustal amphibolites or metagreywackes. The proportion of positive zircon εHf(t) values decreased from Episode #1 to Episode #2, indicating a decreasing tendency of the mantle or juvenile component contribution. Our study suggests that the addition of mantle-derived magmas to the continental crust during oceanic subduction processes played an important role in crustal growth.