Mantle-derived Basaltic Melts Research Articles

Early zircon saturation in adakitic magmatic differentiation series and low Zr content of porphyry copper magmas

Porphyry copper ore-forming magmas worldwide are chemically distinguished from ordinary arc granitoids by lower Zr and by higher Sr/Zr ratios at equivalent SiO2 contents. Low ppm Zr in zircon-saturated melts and high whole-rock Sr/Zr in granitoid samples retaining igneous plagioclase are useful discriminants of Cu-fertile intrusive complexes. These and other chemical discriminants of porphyry-copper-forming melts of tonalite-granodiorite-adamellite composition cannot develop during crystallisation-differentiation at upper-crustal pressures. They indicate unusually high dissolved H2O (≥ 9 wt%) in residual melts at depths near the base of the crust. We compare Zr behaviour during mafic-to-felsic magmatic differentiation in orogenically deforming, copper-mineralised arc segments with behaviour during differentiation in non-orogenic, unmineralised arc segments laterally adjacent along the same subduction zones. In orogenically deforming segments, horizontal compressive stress provides resistance to opening of tensile dyke fractures and promotes entrapment of mantle-derived basaltic melts in magma chambers embedded in hot country rock at Moho-vicinity depths, where magmas cool slowly and residual melts tend to last long enough to experience intermittent chamber replenishment by basaltic melts from the deeper mantle. Over several cycles of replenishment and fractional crystallisation, residual melts acquire high concentrations of inherited chemical components that were largely excluded from cumulus minerals—H2O, Cl, CO2, SO3, etc. Accumulating H2O re-orders the high-pressure crystallisation sequence of igneous minerals in successive cycles and leads to early and prolific production of hornblende and early zircon saturation in mafic melts (< 60 wt% SiO2) of later cycles and leads to low Zr contents of zircon-saturated intermediate-composition residual melts.

Ancient crust-derived syenitic and A-type granitic intrusions in the Emeishan large igneous province, SW China

Within the inner zone of the Emeishan large igneous province (ELIP), SW China, there are several small syenitic and A-type granitic intrusions not spatially related to giant Fe–Ti–V-deposit-bearing mafic–ultramafic layered intrusions. The representative Heime syenitic and Huili granitic intrusions as well as their spatially associated mafic dykes were developed at ca. 260 Ma, contemporaneous with the ELIP. The Heime metaluminous syenites are K-rich and have moderate SiO2 (60.1–61.4 wt%) and high total-alkali (Na2O + K2O = 9.68–10.7 wt%) contents with high K2O/Na2O ratios (1.18 to 1.74). The Huili metaluminous granites are K-rich A-type granites with moderate–high SiO2 (62.7–68.9 wt%), high total-alkali (7.96–8.62 wt%) and Zr (324–853 ppm) contents, and high 10,000 × Ga/Al ratios (2.68–3.22). Both of them are enriched in high-field-strength elements and light rare-earth elements, with significantly negative Nb–Ta–Sr–P–Ti anomalies. They also have highly evolved whole-rock SrNd isotopic compositions (Heime syenites, (87Sr/86Sr)i = 0.7092–0.7105, εNd(t) = −7.77 to −6.09; Huili granites, (87Sr/86Sr)i = 0.7116–0.7146, εNd(t) = −6.34 to −5.32). Zircon δ18O values of the Heime syenites (5.73‰–6.40‰) are slightly higher than those of the mantle zircons, and lower than those of the Huili granites (5.98‰–7.52‰). Zircon Hf 8isotopic compositions of the Huili granites (εHf(t) = −3.30 to +0.73) are higher than those of the Heime syenites (εHf(t) = −7.08 to −2.97), possibly indicating a more contribution of mantle component. In addition, the spatially associated mafic dykes display considerably higher εNd(t) values (Heime mafic dykes, −3.95 to −3.01; Heime mafic dykes, −0.58 to −0.51) than those of the Heime syenites and Huili granites. The parental magmas of both the Heime syenites and the Huili A-type granites show a close affinity to the K-rich, metaluminous syenitic magmas. Based on the Sr–Nd–O–Hf isotopic compositions of the Heime syenites, Huili granites, and spatially associated mafic dykes, we suggest that the K-rich syenitic magmas parental to the Heime syenites were produced mainly by remelting of Paleo–Mesoproterozoic mafic meta-igneous rocks, whereas those parental to the Huili A-type granites were generated by the remelting of Paleo–Mesoproterozoic basement rocks and mixing with small amounts of coeval mantle-derived basaltic melts.

Constraints on the Formation of the Giant Daheishan Porphyry Mo Deposit (NE China) from Whole-Rock and Accessory Mineral Geochemistry

AbstractThere are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. &amp;gt;5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical.

Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

Formation and composition of the Late Cretaceous Gangdese arc lower crust in southern Tibet

Arc lower crust plays a critical role in processing mantle-derived basaltic melts into the intermediate continental crust, yet can only be studied indirectly or in exposed arc sections. Compared with the relatively well-studied oceanic arc sections (e.g., Kohistan and Talkeetna), the composition and formation mechanisms of continental arc lower crust remain less clear. Here we present a geochronological and geochemical study on the Lilong Complex and the Wolong granitoids from the Gangdese arc deep crustal section in southern Tibet. The Lilong Complex is composed of the early (85–95 Ma) mafic-intermediate sequence and late (85–86 Ma) ultramafic sequence. The Lilong crustal section exposed crustal depth extending from ~ 42 to 17 km based on the geobarometry. The mafic-intermediate sequence is a damp (low H2O) igneous differentiation sequence characterized by the subsequent appearance of pyroxene → plagioclase → amphibole → biotite. The ultramafic sequence represents a wet igneous differentiation sequence composed of olivine → pyroxene → amphibole → plagioclase. The 74–84 Ma Wolong granitoids were formed by fractional crystallization of wet magma and intra-crustal assimilation. Calculated seismic properties of the Gangdese deep arc crust are comparable to the average continental crust at a similar depth. The average composition of the Gangdese arc lower crust is basaltic andesite with SiO2 of ~ 54 wt%. The highly incompatible elements in the Gangdese arc lower crust are systematically higher than those of the oceanic arc and are comparable with the estimates of lower continental crust, suggesting continental arc magmatism significantly contributes to the formation of continental crust.

The role of basaltic underplating in the evolution of the lower continental crust

Basaltic underplating revealed by lower crustal xenoliths is usually considered to be manifested by younger zircon ages than those of the pre-existing crust and/or Sr–Nd isotopic heterogeneity resulting from mixing between mantle-derived basaltic melts and crust. The Hannuoba mafic lower crustal xenoliths have long been regarded as a typical example of Mesozoic underplating owing to the presence of 80–160 Ma zircons and evolved Sr–Nd isotopic compositions. However, our integrated study shows that most Mesozoic zircons in the Hannuoba mafic xenoliths precipitated from partial melts derived from the ancient lower crust. Although a few young zircons may record Mesozoic underplating, none of the xenoliths are products of Mesozoic underplating. It provides the first direct evidence that some zircons in lower crustal xenoliths could be exotic. The contrasting O–Hf isotopic compositions of Mesozoic zircons from the Hannuoba lower crustal xenoliths allow us to distinguish zircons that were grown from different hydrous melts from those that represent recrystallized pre-existing zircons. This has major implications for geological interpretation of the age diversity commonly observed in deep-seated xenoliths worldwide. Furthermore, at Hannuoba both the ∼1.8 Ga zircon ages from the granulite terrain and most of the 1.8–1.9 Ga zircon ages from a previously reported banded granulite xenolith also reflect metamorphism rather than underplating. It demonstrates that high-grade metamorphism or partial melting of Archean rocks can result in zircons with significantly younger U–Pb and THfDM ages. We infer that some granulite xenoliths previously regarded as products of Paleoproterozoic basaltic underplating from other regions (e.g., the Wyoming craton and the Siberian craton) may actually be remnants or derivatives of the pre-existing Archean lower crust. The large range in Sr–Nd isotopic compositions for various Hannuoba lower crustal xenoliths is unlikely to have resulted from mixing between basaltic melts and crust but was rather inherited from the ancient lower crust. Most of the Hannuoba mafic xenoliths can be best explained as residues left after partial melting of the late Archean lower crust that may be represented by the granulite terrain to produce the voluminous 125–143 Ma intermediate–felsic magmatic rocks. Therefore, young zircon ages and heterogeneous Sr–Nd isotopic compositions are insufficient criteria to infer recent underplating. Combined with literature data, it shows that Archean mafic granulite xenoliths are widespread globally, thus arguing against previous suggestion that mafic granulites from the lower crust of most Archean cratons might have formed from post-Archean basaltic underplating. It is implied that the role of basaltic underplating in the evolution of the lower crust in many regions may need to be re-evaluated.

Crystal fractionation of contaminated melts and re-melting of newly underplated basaltic crust generated Late Triassic andesitic and dioritic intrusions in the southern Yidun Terrane, SW China

The genesis of andesitic and dioritic rocks in continental arcs is hotly debated, and proposed models include slab melting, juvenile mafic lower-crustal melting, mafic and felsic magma mixing, and mantle-derived melts that underwent a MASH (melting, assimilation, storage, and homogenization) process. The controversy comes from the possibility of their polygenesis and the ambiguity of bulk-rock isotope tracers, not to mention the processes of magma evolution. Here, we report on our investigations in the southern Yidun Terrane (a Triassic continental arc in SW China) into the petrogenesis of two stages of the dioritic Xiuwacu intrusions (phases 1 and 2), the mafic microgranular enclaves (MMEs) hosted by the phase 1 intrusion, as well as the andesitic Hongshan intrusions. We used U-Pb zircon dating, in situ zircon Hf-O isotope analyses, bulk-rock Sr-Nd isotope analyses, and trace element analyses of the zircons from different stages of the intrusions and the MMEs to shed light on their sources and mantle–crust interactions involved in their petrogenesis. Our results suggest that the Xiuwacu MMEs from the phase 1 intrusion crystallized from a melt. Their variable zircon Hf-O isotopic compositions (εHf(t) = +0.4 to +5.3 and δ18O = 4.80‰ to 7.79‰) indicate they were formed by the mixing of mantle-derived basaltic melts and felsic melts. The phase 1 intrusion have an origin similar to that of the MMEs, as indicated by their homogeneous zircon Hf-O isotopic compositions (εHf(t) = +1.3 to +3.9 and δ18O = 6.58‰ to 7.31‰), which fall within the range of the isotopic compositions of the MMEs. The discovery of several inherited zircons and a small recrystallized felsic microgranular enclave hosted by an MME reveals that the MMEs contain relatively more felsic components. Even so, the MMEs still have lower silica contents (62.1–63.4 wt%) than those of the phase 1 intrusion (63.4–70.5 wt%), and this indicates that direct magma mixing of basaltic and felsic melts generated the andesitic MMEs, whereas a crystal fractionation process is required for the dioritic phase 1 intrusion. The porphyritic phase 2 and Hongshan intrusions formed at ~216 Ma, after the phase 1 intrusion, and they had higher oxygen fugacities and crystallization temperatures than the phase 1 intrusion. Given the conclusions of previous researchers that slab breakoff probably occurred at ~216 Ma, we propose that the Xiuwacu phase 2 and Hongshan intrusions formed during the slab breakoff process, the former by partial melting of the newly underplated basaltic lower crust in the MASH zone, and the latter by differentiation of mantle-derived basaltic melts at relatively high pressures.

Multi-stage magmatic plumbing system of the volcano: A case study from Ulleung Island, South Korea

The multi-stage magmatic plumbing system beneath the Ulleung, Dokdo, and Oki-Dogo Island has attracted considerable attention but has seldom been investigated in detail. Herein, we present a comprehensive geochemical analysis of volcanic rocks sampled at the Ulleung Island, including whole–rock major and trace elements as well as Sr and Nd isotopic compositions. Late Cenozoic alkaline volcanic rocks from the Ulleung Island are commonly divided into basaltic rocks (tephrite basanite and basalt) and acidic rocks (trachyte and rhyolite) based on the large petrologic and geochemical variations. All rocks are significantly enriched in light rare-earth elements (REEs) and large-ion lithophile elements and relatively depleted in heavy REEs. The acidic rocks have slightly enriched 87Sr/86Sr and εNd ratios (0.704895–0.705173; −2.48 to −1.91) and distinctly negative Ba, Sr, P, Eu, and Ti anomalies relative to the basaltic rocks (0.704733–0.705006, -2.61– -1.17). A multi-stage magma evolution model was finally developed beneath the Ulleung, Dokdo, and Oki-Dogo Island, characterized by at least two magma reservoirs at different depths approximately ranging between 20 and 100 km. These ascending magmas were retained in a shallower reservoir for ~1.0, ~0.5, and ~4.5 Ma, respectively, prior to eruptions with significant crystal fractionation. Mantle-derived basaltic melts either directly travelled to the surface to erupt as tephrite-basanite and basalt, or were stagnated in shallower reservoirs where they differentiated into acidic magma to erupt as trachyte and rhyolite. The Late Cenozoic Korean Peninsula basalts are mainly divided into two groups, i.e., the depleted mantle (DM) and enriched mantle 1 (EM1) mixing trend beneath the Ulleung and Dokdo Island, and the DM-EM2 mixing connection beneath the Jeju Island, which likely indicates the traversed boundary beneath Korean Peninsula that represents a simple lateral continuation of the boundary between the southeast and northeast China Blocks.

Petrogenesis of Cretaceous (133–84 Ma) intermediate dykes and host granites in southeastern China: Implications for lithospheric extension, continental crustal growth, and geodynamics of Palaeo-Pacific subduction

This paper presents U-Pb zircon geochronology, petrology, and major and trace element, Sr-Nd and zircon Hf isotopic geochemistry of Cretaceous granites and intermediate dykes in the Quanzhou and Xiamen regions of southeastern China. These data are used to investigate igneous petrogenesis and Cretaceous tectonic evolution, and interpret the geodynamics of Palaeo-Pacific slab subduction. Granites in Quanzhou and Xiamen range in age from 133Ma to 87Ma, have high SiO2 and K2O contents, low abundances in P2O5, and an A/CNK index that ranges from 0.97 to 1.09, indicating that they are high-K calc-alkaline metaluminous I-type rocks. Slightly negative ɛ Nd (t) values (−1.2 to −4.4), young Nd model ages (0.87Ga to 1.20Ga) and positive ɛ Hf (t) values (−0.5 to +9.9) of zircon grains indicate that the granites were derived from magmas that melted amphibolite in the middle-lower crust, and which may have assimilated country rocks during emplacement in shallow chambers. The intermediate dykes have no genetic link to the granites and magma mixing was negligible. Eight dyke samples have low SiO2 and high MgO, Ni and Cr contents. Negative ε Nd (t) values (−1.5 to −2.7) and positive ε Hf (t) values (2.7 to 7.6) suggest that the dykes were derived from residual basic lower crust after mafic-crystal accumulation. Two samples of adakite-like dykes are characterised by high Sr/Y ratios (89 to 100) and high SiO2, low K2O, Ni, Cr contents. In combination with slightly negative ε Nd (t) values (−1.7 to −1.8) and positive ε Hf (t) values (2.9 to 4.3), the adakite-like dykes were derived from cumulate basic lower crust which had a mixed source between depleted mantle- and crust-derived melts. Based on our data, combined with previously published work, we suggest that extension-induced middle-lower crustal melting and underplating of mantle-derived basaltic melts were the principal driving mechanisms for Cretaceous granitic magmatism in coastal Fujian Province. Extension was related to subduction retreat whereas steep slab subduction caused underplating of mantle-derived basaltic melts. These processes were coupled and mainly responsible for the tectonic transition during the Cretaceous from compression to extension in the coastal belt of the Cathaysia Plate.

Geochronology and geochemistry of late Jurassic adakitic intrusions and associated porphyry Mo–Cu deposit in the Tongcun area, east China: Implications for metallogenesis and tectonic setting

The genesis of adakites and associated Mo–Cu mineralization in non–arc settings in China is poorly constrained. Here, we present geochronology, geochemistry and Sr–Nd–Pb isotopes for the Tongcun intrusive complex, and report Pb isotopes and 40Ar–39Ar age for the Tongcun Mo–Cu deposit. The Tongcun intrusive complex is composed mainly by granodiorite and monzogranite (phase 1 and phase 2), with emplacement age of 160Ma to 148Ma. The Tongcun complex can be classified as typical high–K calc–alkaline I–type granitoid and also shows adakitic geochemical features. Moreover, the negative Nb, Ta, Ti, and P anomalies and enriched initial 87Sr/86Sr ratios of 0.7083–0.7092 of the Tongcun intrusive complex are consistent with those of the subduction–related magmatism. The 40Ar–39Ar dating of sericite, which is intergrown with chalcopyrite, indicates that the late Cu mineralization event occurred at ~155.5Ma. The early Mo (Cu) and the late Cu mineralization events in this deposit were temporally, spatially and genetically associated with the emplacement of monzogranite (phase 1). There are no obvious linear correlation between SiO2 and most of the major and trace elements, and all rock samples fall within the fields of unfractional crystallization felsic granites in Zr+Nb+Ce+Y versus FeOT/MgO and (K2O+Na2O)/CaO diagrams, indicating that partial melting rather than fractional crystallization has played an important role for the formation of the Tongcun intrusive complex. Magmatic inherited zircons from the Tongcun granitoids with the age peaked at 780–812Ma, imply that the Neoproterozoic igneous rocks in the lower crust have been incorporated into the magma source. The uniform εNd(t) (−6.3 to −7.3), initial 87Sr/86Sr, 207Pb/204Pb (15.596–15.621), and 208Pb/204Pb (38.374–38.650), as well as high K2O contents (3.36–4.10wt.%) and relatively high Mg# values (35.40 to 40.30) suggest the Tongcun intrusive complex was derived from partial melting of the thickened lower continental crust triggered by basaltic magma underplating plus additional input from the EM II mantle-derived basaltic melts. The Tongcun area was controlled by a compression setting related to the subduction of the Paleo–Pacific Plate in Mesozoic period.

Crustal melting and magma mixing in a continental arc setting: Evidence from the Yaloman intrusive complex in the Gorny Altai terrane, Central Asian Orogenic Belt

Granitoids and their hosted mafic enclaves may retain important information on crust–mantle interaction, and thus are significant for study of crustal growth and differentiation. An integrated petrological, geochronological and geochemical study on the granitoid plutons of the Yaloman intrusive complex from the Gorny Altai terrane, northwestern Central Asian Orogenic Belt, was conducted to determine their source nature, petrogenesis and geodynamics. Mafic enclaves are common in the plutons, and a zircon U–Pb age (389Ma±4Ma) indicates that they are coeval with their granitoid hosts (ca. 393–387Ma). Petrographic observations reveal that these mafic enclaves probably represent magmatic globules commingled with their host magmas. The relatively low SiO2 contents (46.0–60.7wt.%) and high Mg# (38.9–56.5) further suggest that mantle-derived mafic melts served as a crucial component in the formation of these mafic enclaves. The granitoid hosts, including quartz diorites and granodiorites, are I-type in origin, possessing higher SiO2 contents (60.2–69.9wt.%) and lower Mg# (32.0–44.2). Their zircon Hf and whole-rock Nd isotopic compositions indicate that the magmas were dominated by remelting of Neoproterozoic (0.79–1.07Ga) crustal materials. Meanwhile, the geochemical modeling, together with the common occurrence of igneous mafic enclaves and the observation of reversely zoned plagioclases, suggests that magma mixing possibly contributed significantly to the geochemical variation of the granitoid hosts. Our results imply that mafic magmas from the mantle not only provided substantial heat to melt the lower crust, but also mixed with the crust-derived melts to form the diverse granitoids.The oxidizing and water-enriched properties inferred from the mineral assemblages and compositions imply that the granitoid plutons of the Yaloman intrusive complex were possibly formed in a continental arc-related setting, which is also supported by their geochemistry. The Devonian granitoids from the Gorny Altai terrane show remarkable temporal–spatial–petrogenetic affinities to the counterparts from the Altai-Mongolian terrane, indicating that these two terranes were possibly under subduction of the same oceanic plate (i.e., the Ob-Zaisan Ocean). The voluminous granitoids signify significant crustal recycling and growth as a response to the underplating of extensive mantle-derived basaltic melts.

Transition from adakitic to bimodal magmatism induced by the paleo-Pacific plate subduction and slab rollback beneath SE China: Evidence from petrogenesis and tectonic setting of the dike swarms

The late Mesozoic magmatic record of SE China is dominated by felsic volcanics and intrusions. However, this magmatism mainly occurred in coastal areas at 110–80 Ma, in contrast to poorly researched dike swarms that were emplaced inland at 165–120 Ma. Here, we focus on Early Cretaceous mafic and felsic dike swarms that provide new insights into the tectono-magmatic evolution of SE China. The swarms were intruded into Neoproterozoic plutons and include granodioritic porphyry, granitic porphyry, and diabase dikes. The granodioritic porphyry (128 ± 2 Ma) dikes are geochemically similar to adakitic rocks, suggesting that inland adakitic magmatism occurred between ca. 175 and ca. 130 Ma. The majority of these adakitic rocks are calc-alkaline and have Sr-Nd-Hf-O isotopic compositions that are indicative of derivation from a Neoproterozoic magmatic arc source within the lower crust. The granitic porphyry and diabase dikes were emplaced coevally at ca. 130 Ma, and the former contain high alkali and high field strength element (HFSE; e.g., Zr, Nb, Ce, and Y) concentrations that together with their high Ga/Al and FeOT/(FeOT + MgO) ratios imply an A-type affinity. The widespread ca. 130 Ma magmatism that formed the A-type granites and coeval diabase dikes defines a NE-SW trending inland belt of bimodal magmatism in SE China. The presence of mafic enclaves in some of the A-type granites, and the Sr-Nd-Hf isotopic compositions of the latter are indicative of inadequate mixing between the basement sediment-derived and coeval mantle-derived basaltic melts that define the bimodal magmatism. The transition from adakitic rocks to bimodal magmatism in the inland region of SE China indicates a change in the prevailing tectonic regime. This change was associated with an increase in the dip angle of the northwestward-subducting paleo-Pacific Plate beneath SE China between the Middle Jurassic and the Early Cretaceous. This resulted in a transition from a local intra-plate extensional tectonic regime induced by far-distance stress at the plate margin to a back-arc extensional regime induced by rollback of the subducted slab.

Late Mesozoic molybdenum mineralization on Hainan Island, South China: Geochemistry, geochronology and geodynamic setting

Hainan Island has become an important molybdenum (Mo) producer in East China, with the proven and measured metal Mo reserves of >0.29Mt and >0.69Mt, respectively. The known Mo-related deposits and occurrence which genetically might be attributed to porphyry- and hydrothermal vein types are hosted predominantly within the Cretaceous granitoids. These granitoids belong to high-K calc-alkaline, metaluminous to weakly peraluminous I-type series, and have geochemical affinities to arc-related basalts as defined by SiO2 (57.40–77.34wt.%) and K2O (2.45–6.28wt.%) contents, A/CNK ratios (mostly 0.86–1.10), depleted K, Ba, Nb, Sr, P and Ti, and enriched Rb, Th, Pb, Zr and La, and LREE-enriched REE patterns (La/YbN=7.1–66.3) with pronounced negative Eu anomalies to no Eu anomalies (Eu/Eu*=0.35–1.08). The highly variable geochemical compositions might be linked to magmatic processes. The LA-ICP-MS U–Pb dating on zircon have revealed three stages of magma emplacement for the Cretaceous Mo-hosting granitoids during ca. 113–108Ma, ca. 100–94Ma and ca. 90–70Ma, respectively. The initial 87Sr/86Sr ratios (from 0.70567 to 0.71208) and ɛNd(t) values (from −3.9 to −6.9) suggest that the host granitoids were sourced from an enriched subcontinental lithospheric mantle that had been metasomatized by fluids and/or melts released from ancient, subducted oceanic crust under an extension-induced setting. This setting is most favorable to variable degree of magma mixing between mantle-derived and crust-derived melts.With the second event as most important and the third event likely recording the youngest Mo mineralization in East China, the Re–Os dating of molybdenite from two types of Mo deposits on Hainan Island has identified three Mo mineralizing events occurring at ca. 112Ma, ca. 106–95Ma and ca. 89–72Ma. They are consistent with the episodes of the Cretaceous magmatism. The Re concentrations of molybdenites and δ34S values of sulfides (1.36–5.73‰ and average 4.3‰) commonly indicate that the metal Mo is of hybrid origin between mantle and crust but with variable degree of hybridization. Given that the subduction of the Paleo-Pacific plate beneath the eastern Asian margin dominated the Late Mesozoic tectonism of South China, and that Hainan Island has been a part of the Cathaysia Block since Late Permian, the widespread Cretaceous extension and associated Mo mineralizing event(s) on Hainan Island might be a response to episodic rollback of the subducted slab, which led to the underplating of mantle-derived basaltic melts and associated deep crustal melting. Moreover, this tectono-magmatic scenario likely resulted in the migratory pluses of Mo metallogenesis and the change of metal Mo sources from northeast to southwest Hainan Island.

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia, central Anatolia, Turkey): age, petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr–Nd–Pb and δ18O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Nigde Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by 87Sr/86Sr = 0.7038, 143Nd/144Nd = 0.5128, 206Pb/204Pb = 18.80, 207Pb/204Pb = 15.60 and 208Pb/204Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of 143Nd/144Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition (206Pb/204Pb = 18.84–18.87, 207Pb/204Pb = 15.64–15.67 and 208Pb/204Pb = 38.93–38.99). 87Sr/86Sr isotopic compositions of mafic (0.7038–0.7053) and felsic (0.7040–0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ18O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between 87Sr/86Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High 87Sr/86Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant 87Sr/86Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.

Crustal growth at active continental margins: Numerical modeling

The dynamics and melt sources for crustal growth at active continental margins are analyzed by using a 2D coupled petrological–thermomechanical numerical model of an oceanic-continental subduction zone. This model includes spontaneous slab retreat and bending, dehydration of subducted crust, aqueous fluid transport, partial melting, melt extraction and melt emplacement in form of extrusive volcanics and intrusive plutons. We could identify the following three geodynamic regimes of crustal growth: (i) stable arcs, (ii) compressional arcs with plume development, and (iii) extensional arcs. Crustal growth in a stable subduction setting results in the emplacement of flattened intrusions in the lower crust. At first dacitic melts, extracted from partially molten rocks located atop the slab (gabbros and basalts), intrude into the lower crust followed by mantle-derived (wet peridotite) basaltic melts from the mantle wedge. Thus extending plutons form in the lower crust, characterized by a successively increasing mantle component and low magmatic addition rates (10km3/km/Myrs). Compressional arcs are accomplished by the formation and emplacement of hybrid plumes. In the course of subduction localization and partial melting of basalts and sediments along the slab induces Rayleigh Taylor instabilities. Hence, buoyant plumes are formed, composed of partially molten sediments and basalts of the oceanic crust. Subsequently, these plumes ascend, crosscutting the lithosphere before they finally crystallize within the upper crust in form of silicic intrusions. Additionally, intrusions are formed in the lower crust derived by partial melting of rocks located atop the slab (basalts, gabbros, wet peridotite) and inside the plume (basalts, sediments). Magmatic addition rates are somewhat higher compared to stable arcs (40–70km3/km/Myrs). Subduction in an extensional arc setting results in decompression melting of dry peridotite. The backward motion of the subduction zone relative to the motion of the plate leads to thinning of the overriding plate. Thus, hot and dry asthenosphere rises into the neck as the slab retreats, triggering decompression melting of dry peridotite. Consequently large volumes of mafic (oceanic) crust are formed in the backarc region with total magmatic addition rates being as high as 90–170km3/km/Myrs.

Nd‐Sr Isotopic Geochemistry of the Late Archean‐Paleoproterozoic Granitoids in the Lüliang‐Wutai Terrain, North China Craton, and Implications for Petrogenesis

Abstract: In this paper we report geochemical and Nd‐Sr isotopic data for a late Archean gneissic granitic pluton (Hengling pluton), an early Paleoproterozoic complex (Xipan complex) and a late Paleoproterozoic granitic pluton (Yunzhongshan granites) from the Lüliang‐Wutai terrain, North China, to trace the source of these late Archean‐Paleoproterozoic granitoids and, particularly, to understand the nature and mechanism of continental growth at that time. The Hengling granitic gneisses (ca. 2.51 Ga) are characterized by high Na2O and LILEs, TTG‐like REE patterns (highly depleted HREE and minor Eu anomalies) and moderately depleted Nd‐Sr isotopic compositions (∍Nd(t) =1.2‐2.7, ISr=0.7015‐0.7019), and were considered as being products of arc magmatism that was developed upon the North China craton. The Xipan complex (ca. 2.2 Ga) contain gabbroic diorite and monzonite, mostly being Na2O‐rich, highly fractionated REE patterns and isotopically enriched (∍Nd(t) =−1.5 to −4.1, ISr=0.7038‐0.706). The gabbroic diorites probably originated from melting of an enriched mantle source, but significantly contaminated by lower crustal material, and the monzonites probably represent a product of a mixture between the gabbroic dioritic magma and granitic melts of crustal origin. The Yunzhongshan post‐collisional granitoids (ca. 1.8 Ga) are characterized by high‐K affinity and highly‐enriched and homogeneous Nd isotopic compositions (∍Nd(t) = −4.9 to −5.7), although they split into two groups in terms of REE patterns: one group showing elevated HREE (and Sc, Y and Zr) with significant negative Eu anomalies and the other showing highly depleted HREE and, to a lesser extent, mid‐REE with negligible Eu anomalies. These granites are genetically related to a process of extensional collapse of a thickened orogen. They formed through magma mixing between mantle‐derived basaltic magmas and crust‐derived granitic melts, followed by significant fractionation of ferromagnesian phases (like hornblende and Cpx) and feldspar and accessory zircons. Some Yunzhongshan granites show very old Nd model ages (2.9‐3.0 Ga), suggesting the existence of continental crust older than 2.7 Ga, which is supported by our zircon Hf isotopic data for these granites.

Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: A response to tectonic evolution

This paper summarizes the new results on the petrogenesis of Mesozoic granitoids and volcanic rocks in South China. The authors propose that these rocks were formed in time and space as a response to regional tectonic regime change from the continent-continent collision of the Indosinian orogeny within the broad Tethyan orogenic domain in the Early Mesozoic (T1–T3) (Period I) to the largely extensional setting as a result of the Yanshanian orogeny genetically associated with the NW–WNW-ward subduction of the paleo-Pacific oceanic lithosphere in the Late Mesozoic (J2–K2) (Period II). Of the Period I Indosinian granitoids, the early (T1–T21) ones are syn-collisional, and formed in a compressional setting; the late (T22–T3) ones are late-collisional, and formed in a locally extensional environment. During the Period II Yanshanian magmatism, the Early Yanshanian (J2–J3) granitoid-volcanic rocks, which are distributed mainly in the Nanling Range and in the interior of the South China tectonic block (SCB), are characteristic of rift-type intraplate magmatism, whereas the Late Yanshanian K1 granitoid-volcanic rocks are interpreted as genetically representing active continental margin magmatism. The K2 tholeiitic basalts interlayered with red beds are interpreted as genetically associated with the development of back-arc extensional basins in the interior of the SCB. The Yanshanian granitoid-volcanic rocks are distributed widely in South China, reflecting extensional tectonics within much of the SCB. The extension-induced deep crustal melting and underplating of mantle-derived basaltic melts are suggested as the two principal driving mechanisms for the Yanshanian granitic magmatism in South China.

Variscan basic dykes in the Pelagonian (Northern Greece and south FYROM): Geodynamic significance based on petrological, geochemical and geochronological studies

Dolerite dykes intruding Variscan plutonites were studied in terms of mineralogy, petrology, geochemistry and geochronology. The main mineral constituents were studied and the sequence of crystallization has been derived. The geochemical characteristic indicate mantle origin of the dolerites and magma sources different from the hosting granitoids. From SHRIMP analyses of five spots on four different zircon crystals, resulted a 292.0±4.1 Ma age that is interpreted as the time of crystallization of the dolerite. The hosting granitoids are probably the result of mixing between two possible end-members: enriched mantle and acid metaigneous or lower crustal metasediments. The Variscan age of the dolerites, in combination with the geochemical characteristics, indicated that the enriched mantle basaltic material should be the source of the dolerite veins. These mantle-derived basaltic melts may represent the underplated material, which probably provided the necessary thermal input to the dehydration melting in the lower crust. The dolerites should have intruded the newly formed batholiths before or at the first stages of their uplift, recording the last events of the Variscan subduction.

Isotopic constraints on the origin of AMCG-suite rocks on the Lofoten Islands, N Norway

¶Sm–Nd, Rb–Sr and Pb–Pb isotopic compositions of 34 intrusive AMCG (anorthosite–mangerite–charnockite–granite) suite rocks and spatially related ferrodiorites and gabbros from the Lofoten Islands, northern Norway, suggest that almost all Lofoten intrusive rocks can be modelled as a mixture of mainly two components: Archean lower crustal material and an about 1.8 Ga mantle-derived component. Isotopically, the gabbros and anorthosites overlap the mangeritic rocks in all three isotope systems. Hence, the isotopic data are in agreement with a model that relates the mangeritic rocks to the fractionation of crustally contaminated mantle-derived basaltic melts. Overlap in all three isotopic systems indicates that at least part of the assimilation process predates or accompanies fractionation. Whole rock geochemistry supports this model: based on major, minor, and trace element data, primitive 1.79–1.86 Ga mangerites could have formed from anorthosite residual liquids – the ferrodiorites – by fractionation processes involving Fe–Ti oxides, Fe–Mg silicates and apatite combined with contamination of the ferrodiorites with about 50 wt.% crustal anatectic melt.

Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 Ma (Early Cretaceous). It occurred about 5 to 10 m.y. after the ∼165 to 159 Ma Josephine ophiolite was thrust beneath older parts of the province during the Nevadan orogeny (thrusting from ∼155 to 148 Ma). The magmatism also corresponds to a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist almost entirely of tonalite and trondhjemite. Poorly-defined concentric zoning is common. Tonalitic rocks are typically of the low-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite. The suite is characterized by low abundances of K2O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate (∼450 ppm) by comparison with Sr-rich arc lavas interpreted to be slab melts (up to 2000 ppm). Initial 87Sr/86Sr, δ18O, and ɛNd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual assemblage of clinopyroxene+garnet±plagioclase±amphibole; residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source. Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low- and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary.