Underplating Of Mafic Magmas Research Articles

Middle Cambrian granites in the Dunhuang Block (NW China) mark the early subduction of the southernmost Paleo-Asian Ocean

The formation and evolution of the Central Asian Orogenic Belt (CAOB) were closely associated with the opening and closure of the so-called “Paleo-Asian Ocean” (PAO). The subduction of the southern PAO is expected to have resulted in widespread Paleozoic arc magmatism in the southernmost CAOB. However, the precise location and timing of such subduction remain uncertain. The Dunhuang Block (DHB) in the southernmost CAOB is thought to have recorded such subduction. In this contribution, we report the results of the zircon U–Pb ages, Hf isotopes, and whole-rock geochemical data for the Lianghu, Dapokouzi, and Xiaowan plutons in the northeastern DHB, NW China. Zircon U–Pb dating reveals that these plutons formed at ca. 510 Ma (the Middle Cambrian) and may represent the oldest arc-related magmatism in the DHB. Geochemically, the Dapokouzi pluton is characterized by high SiO2 (70.7–71.2 wt%), Na2O (5.11–5.26 wt%), Al2O3 (15.5–15.6 wt%), and Sr (730–733 ppm) contents, high Sr/Y (380–398) ratios, moderate Mg# (49–50) values, low MgO (0.61 wt%), K2O (2.00–2.01 wt%) and Y (1.84–1.93 ppm) contents, and weakly positive Eu (1.39–1.44) anomalies, indicating geochemical affinities of adakitic rocks. They have positive εHf(t) values (+11.2 to +13.9) close to the depleted mantle line and young two-stage model ages (TDM2 = 571–742 Ma). All the geochemical characteristics show that the Dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle-derived materials and a residue phase dominated by garnet-bearing amphibolite. Similarly, the Lianghu and Xiaowan plutons have high SiO2 (70.2–74.4 wt%) and Al2O3 (12.8–14.3 wt%), low MgO (0.61–1.01 wt%) with Mg# values of 35–43 and K2O (0.43–1.55 wt%) contents. Compared to the Dapokouzi pluton, they exhibit low Sr (121–155 ppm) and Y (15.3–23.1 ppm) contents, low Sr/Y (5.25–9.54) ratios and Cr-Ni-Co concentrations, and pronouncedly positive εHf(t) values (+11.0 to +14.7), indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths. Data from this and previous studies convincingly suggest the DHB was located in a thickened continental arc setting during the early Paleozoic, probably related to the early subduction of the PAO lithosphere. Crustal thickening, as indicated by the geochemical signature of the Dapokouzi pluton, was most likely associated with the underplating of mantle-derived mafic magmas during the slab subduction. This study therefore highlights that the subduction of the southern PAO might have initiated prior to 510 Ma, possibly dating back to the early Cambrian.

Deciphering the Sardic (Ordovician) and Variscan deformations in the Eastern Pyrenees, SW Europe

Detailed geological mapping of the La Cerdanya area (Canigó unit, Eastern Pyrenees) provides new data characterizing the different structural styles exhibited by Cambrian–Lower Ordovician (Jujols Group) and Upper Ordovician successions. Their unconformable contact, related to the Sardic Phase, ranges from 0° (paraconformity) to 90° (angular discordance). The Jujols Group rocks topped by the unconformity are affected by Sardic foliation-free open folds. The pre-Sardic succession, the Sardic Unconformity and the lower part of the post-Sardic succession (Rabassa Conglomerate and Cava formations) are cut and offset by several Late Ordovician NNE–SSW-trending synsedimentary extensional faults associated with hydrothermal activity, which dramatically affected the thickness of the lower part of the Upper Ordovician succession. We relate (1) the Mid-Ordovician Sardic uplift and erosion, and (2) a Mid- to Late-Ordovician upward propagating extensional fault system bounding the outline of half-grabens, subsequently infilled by alluvial deposits, to a thermal doming event (about 475–450 Ma) that led to the uplift and stretching of the Ordovician lithosphere. Thermal doming may be caused by mafic magma underplating and responsible for the coeval calc-alkaline magmatic activity broadly developed in the Eastern Pyrenees. We discuss the similarities between the Mid-Ordovician Sardic Unconformity and other Early Paleozoic unconformities described in neighbouring areas. Finally, we suggest a geodynamic scenario in which a regional-scale thermal event was related to the opening of the Rheic Ocean that led to the drift of Avalonia from the SW European margin of Gondwana.

Strongly Peraluminous Granites across the Archean–Proterozoic Transition

AbstractStrongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

Neoproterozoic S-type granites in the western Jiangnan Orogenic Belt, South China: Implications for petrogenesis and geodynamic significance

ABSTRACT The Neoproterozoic Jiangnan Orogenic Belt in south China was formed by the amalgamation of the Yangtze and Cathaysia blocks, however, the timing of its formation is still debated. In this study, we present whole rock geochemistry, U-Pb zircon ages, Lu-Hf and Nd-isotope systematics on the Neoproterozoic Yexijiang and Baomuping granites occurring in the western segment of the Jiangnan Orogenic Belt. These granites mark the initiation of post-collision extension environment. Zircon U-Pb age data identify 807–805 Ma emplacement age for these granites. They have low and variable ΣREE concentrations (47–115 ppm) and show moderate to highly fractionated REE patterns ([La/Sm]N = 2.83–3.78) with low [Gd/Yb]N (1.34–2.31) and negative Eu anomalies (Eu* = 0.34–0.57). Geochemical features and presence of peraluminous minerals classify them as S-type granites and P-T parameters indicate low to moderate temperature (730–791 °C) and pressure (middle crustal level) conditions. These granites show consistent negative whole rock ɛNd(t) (−5.7 to −6.6) and magmatic zircon ɛHf(t) values from −0.3 to −3.8 while inherited zircon grains show variable ɛHf(t) values (+2.1 to −13.0) and two-stage Hf model ages (1.80 to 2.88 Ga). In particular, one Neoproterozoic (899 Ma) inherited zircon grain displays depleted Hf [ɛHf(t) = +5.4 and TDM2 = 1.34 Ga] isotopic characteristic. These lines of evidence indicate a sedimentary source for Yexijiang and Baomuping granites, possibly the Archean to Neoproterozoic crustal rocks, and emplacement in a post-collision tectonic environment triggered by mafic magma underplating during the amalgamation of the Yangtze and Cathaysia blocks. We infer that convergence in the Jiangnan Orogenic Belt occurred during 830–800 Ma therefore, unrelated to the global Grenvillian Orogeny.

Late Mesozoic magmatism in the East Qinling Orogen, China and its tectonic implications

The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic, with concomitant voluminous granitoids formation. In this study, we present results from petrological, geochemical, zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen. We also compile published geochronological, geochemical, and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps. The newly obtained age data yield two group of ages at ∼145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes, with the ∼145 Ma interpreted as response to the peak of magmatism in the region, and the ∼140 Ma as the timing of formation of the felsic dykes. The corresponding Hf isotopic data of the granite porphyries display negative εHf(t) values of –16.67 to –4.61, and Hf crustal model ages (TDMC) of 2255–1490 Ma, indicating magma sourced from the melting of Paleo- to Mesoproterozoic crustal materials. The compiled age data display two major magmatic pulses at 160–130 Ma and 111–108 Ma with magmatic quiescence in between, and the zircon Hf isotopic data display εHf(t) values ranging from –41.9 to 2.1 and TDMC values of 3387–1033 Ma, suggesting mixed crustal and mantle-derived components in the magma source, and correspond to multiple tectonic events during the Late Mesozoic. The Luanchuan granitoids are identified as I-type granites and most of these are highly fractionated granites, involving magma mixing and mingling and crystal fractionation. The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting, with E–W extension in the Early Cretaceous. This extension is correlated with the N–S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E–W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction, eventually leading to lithospheric thinning, asthenospheric upwelling, mafic magma underplating, and crustal melting in the East Qinling Orogen.

Neoarchean granitoid gneisses in Eastern Hebei, North China Craton: Revisited

Neoarchean granitoid gneisses are widely distributed throughout Eastern Hebei, eastern North China Craton, which are dominated by deformed and metamorphosed tonalite–trondhjemite–granodiorite (TTG), diorite, and granite. This study presents the results of systematic zircon U–Pb geochronological, whole-rock geochemical and Sm–Nd isotopic analyses of the Neoarchean granitoid gneisses in Eastern Hebei. These data provide insights into the Archean–Paleoproterozoic multiple tectonothermal events as well as the petrogenesis of these gneisses. Zircon U–Pb isotopic dating indicate that the granitoid gneisses were contemporaneously emplaced between 2546 ± 10 and 2510 ± 10 Ma, reflecting a giant Neoarchean igneous event throughout Eastern Hebei. Subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2.48–2.45 Ga. The close spatial and temporal relationships in terms of magmatism and metamorphism at ca. 2.5 Ga suggest a uniform tectonothermal evolution of Eastern Hebei. The granitic gneisses have mainly originated from the partial melting of juvenile metamorphosed greywackes, with minor involvement of basalts. The large geochemical and isotopic variations within the dioritic and TTG gneisses both provide evidence for the mixing of mafic and felsic magmas, coupled with fractional crystallization. However, the chemical differences between the dioritic and TTG gneisses might result from individual mafic magma sources, viz. basaltic and high-Mg melts. The mafic magma may also form the basalt or komatiite within the greenstone belt or evolve via fractional crystallization prior to the magma mixing. Large-scale granitoid activities were possibly related to mafic magma underplating. The combined geochronological, geochemical, and geological data support an Archean proto-mantle plume model for a geodynamic interpretation of the eastern North China Craton during the Neoarchean.

Geochronology and geochemistry of Early Silurian felsic volcanic rocks in the Dabaoshan ore district, South China: Implications for the petrogenesis and geodynamic setting

The Early Paleozoic orogeny in South China has remained controversial in the long run with debate over intracontinental orogeny versus plate subduction. Limited studies have focused on the Early Paleozoic volcanic rocks in South China, which is crucial for us to understand the tectonic evolution of the South China Block. The Dabaoshan Fe–Cu–Pb–Zn–Mo–W orefield, one of the largest polymetallic deposits in South China, is hosted by Paleozoic volcanic‐sedimentary rocks and Mesozoic granitic porphyry. In this study, zircon U–Pb dating yielded ages of 439.4 ± 2.7 Ma, 439.3 ± 3.2 Ma, and 442.0 ± 2.2 Ma for the Dabaoshan dacite. These ages are all in good agreement within errors and indicate that the dacite formed during the early Silurian (ca. 440 Ma). The dacites are slightly peraluminous, calc‐alkaline with relatively high K2O + Na2O, FeOt, TiO2, CaO/Na2O ratios, and small negative Eu anomalies. They show enrichment in LILEs and LREE and depletion in Ba, Nb, Sr, P, and Ti. It has high initial 87Sr/86Sr values (0.70983–0.71140) and negative εNd(t) values ranging from −7.6 to −10 with T2DM ages of 1.80–1.99 Ga, εHf(t) values from −4.95 to −15.48 with T2DM ages of 1.72 Ga to 2.38 Ga. The overlapped Nd–Hf two‐stage model ages range from 1.89 Ga to 1.99 Ga, which indicates that the Dabaoshan dacite originated from a Paleoproterozoic source. Combining these geochemical and in situ zircon Hf isotopic data, we propose that the Dabaoshan dacite may have originated from the partial melting of mixed source of Paleoproterozoic metasedimentary and metaigneous rocks as a result of underplating of mafic magma during the post‐orogenic collapse and extension followed by lithospheric thining and asthenospheric mantle upwelling in an intraplate setting. Based on field observation and the geochronology results, we propose that the stratiform Cu–Pb–Zn mineralization may be related with the early Silurian volcanic rocks.

The different sources and petrogenesis of Jurassic intrusive rocks in the southern Lhasa subterrane, Tibet: Evidence from the trace element compositions of zircon, apatite, and titanite

ABSTRACT The Jurassic magmatism distributed in the southern Lhasa subterrane (SLT) has been recognized as a record of the early-stage evolution of the Neo-Tethys ocean; however, the magma compositions and detailed crystallization histories remain an enigma. Zircon, apatite, and titanite in igneous rocks can serve as effective petrogenetic indicators. In this paper, we present the whole-rock geochemical, zircon U Pb data, zircon Hf isotopic analyses, and major and trace elemental compositions of zircon, apatite, and titanite from Baiga granite porphyry (EGP), Shanba granodiorite (EGD), Pengcuolin granodiorite porphyry (MGP), and Dazhuqu monzonite (LM) rocks in the SLT to constrain their source and petrogenesis. The results from this study show that a combination of zircon Ti, Th/U, Hf, and rare earth element (REE) contents, apatite halogen, REE and Sr contents, and titanite REEs can be used to track the detailed magmatic crystallization histories. Higher Sr contents are found in apatite (SrAp) from EGP (ca. 192 Ma) and EGD (ca. 176 Ma) than those in their host rocks (SrWR), indicating that their magma genesis may have involved some mafic magma. The magma genesis of LM (ca. 159 Ma) may have involved more reduced mafic (or least evolved) magma, as evidenced by the higher Th/U but lower Yb/Gd ratios, higher zircon saturation and Ti-in-zircon temperatures in zircon, highest SrAp/SrWR ratios, and large positive Eu anomalies in titanite from LM. The lower Cl contents and higher (La/Sm)N values in apatite from MGP (ca. 172 Ma) are inherited from the low-Cl melts. The MGP rocks have higher zircon Ce4+/Ce3+ and EuN/EuN* ratios, a greater apatite SO3 content, and a higher titanite total REE content, which indicates that MGP has a higher oxidation state than do EGP, EGD, and LM. The MGP also exhibits a higher H2O content (>11 wt%) dissolved in the melt, greater Ba/La and Nb/Ta ratios in whole rock, and higher Nb/Ta ratios in zircon. However, MGP demonstrates lower zircon saturation and Ti-in-zircon temperatures, which suggest that MGP was generated by fluid-fluxed melting below 750 °C. The low Ti-in-zircon temperatures but high La/Yb and Sr/Y ratios of MGP indicate that a local thickened crust likely existed in the Middle Jurassic period in the SLT due to mafic magma underplating during this time.

Petrogenesis and Geodynamic Implications of Late Jurassic Diorite Porphyry in the Neoproterozoic Ophiolitic Mélange of NE Jiangxi (South China)

Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U–Pb dating, Hf isotopes and whole‐rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U–Pb dating by SIMS and LA‐ICP‐MS methods yielded an emplacement age of ∼160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low SiO2 (58.08wt%–59.15wt%), and high Na2O (5.00wt%–5.21wt%) and MgO (4.60wt%–5.24wt%) contents with low TFeO/MgO ratios (1.02–1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios (24.8–31.1) and high Y contents (14.6–18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions (εHf(t)=−6.1‐ −0.1; TDM2=1597–1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust (i.e., early Neoproterozoic mafic arc‐related rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal‐spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat‐slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat‐slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.

Petrogenesis and mineralization of the porphyry and skarn mineralization-related Husite intrusion in the Boluokenu metallogenic belt, Western Tianshan Orogenic Belt, NW China

Many porphyry and skarn Cu (Fe, Mo, Au, Zn) deposits have been discovered along the western Tianshan Orogenic Belt within the Central Asian Orogenic Belt. Generally, these Cu polymetallic deposits are considered to be linked to porphyries that formed in an arc setting, e.g., in a continental magmatic arc and island magmatic arc, but an agreement on their source region has not been reached. The Husite granitic complex and associated Kekesala skarn Fe-Cu deposit from the Boluokenu metallogenic belt provide significant insights into a comprehensive study of the ore-forming porphyries in Boluokenu. The ore-forming porphyries are mainly granodiorites and belong to calc-alkaline and high-K calc-alkaline series. LA-ICP-MS zircon U-Pb dating on the granodiorites yields ages of 368 Ma. The trace element compositions of the granodiorites are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies and are similar to rocks that form in a continental arc setting. These rocks also show large variations in their εHf (t) (+4.1 to +9.7) and Mg# (38–69) values, alongside low Ba/Th (4–230) and high Th/Yb (3.77–7.41) and Th/La (0.21–0.71) ratios, suggesting that the granodiorite magma originated from the partial melting of mafic lower crust with a mantle wedge component and involved signals of subduction-zone sediments and fluids. The Husite mafic microgranular enclaves (MMEs) exhibit high Mg# (50–62), MgO (3.25–6.93 wt%) and low SiO2 (48.8–59.5%) contents, with positive εHf(t) from +3.3 to +7.4. The zircon LA-ICP-MS U-Pb ages of the MMEs (373 Ma) indicate nearly coeval formation between the MMEs and HMPs during the Late Devonian. These features with the petrological and mineralogical characteristics (such as needle-like apatite and disequilibrium feldspar xenocrysts) suggest that the Husite MMEs formed from the mixing of felsic magma from the partial melting of mafic lower crust and mafic magma that was derived from mantle wedge, rather than restite, xenoliths or fractional crystallization of magma. Thus, the skarn Fe-Cu mineralization were probably related to hybrid intermediate-acid magma that is characterized by high water and oxygen contents and high sulfur fugacity. It can be inferred that the response to ongoing slab roll-back during the Junggar plate’s subduction beneath the Yili Block, including the underplating of mantle-derived mafic magmas from the metasomatism of subduction-zone melts, led to the partial melting of the mafic lower crust to form felsic magma. Then, the continuous reaction between the mafic and felsic magmas further increased the Fe-Cu concentrations and was closely related to the subsequent Fe-Cu mineralization.

Geochemistry and chronology of the granites in Alasituo, west Tianshan Orogen: Implications for a magma mixing origin

There are intense magmatic activities in the Alasituo integrated gold exploration area, west Tianshan Orogen. On the basis of field characteristics, geochemistry features, and chronology in Alasituo, the formation mechanism of complex granite rocks was explored. The intrusive rocks are divided into 3 parts: a silicic endmember, a mafic endmember, and intermediate rocks. Mafic microgranular enclaves, as well as spotted structure, silicic veins intruding mafic rocks and plagioclase xenocrysts, developed in rocks. Under the microscope, there are many unbalanced mineral combinations, needle‐like apatites, and poikilitic structures. Futhermore, there is an obvious linear trend between the mafic endmember samples, intermediate samples, and the silicic endmember samples in Harker diagrams. All samples were evolved from magma mixing in TFe2O3‐MgO diagram. The trace element concentrations of intermediate rocks have the transitional features of both silicic and mafic endmember rocks. The above characteristics show that rocks have experienced low degree and uneven physical and chemical magma mixing. The silicic endmember rocks have a high content of K2O + Na2O, Zr, Nb, Y, and REE; high Ga/Al ratios, and; low content of Sr, Eu, Ti, and P, which have the characteristics of A‐type granites with a crustal source. The high content of MgO, TFe2O3, and low K2O/Na2O in mafic endmember rocks indicate a mantle source. The LA‐ICP‐MS U–Pb zircon age of the granodiorites is 350.3 ± 3.0 Ma. Combined with regional geological background and geochemical characteristics, the intrusive rocks were formed when the thinning lithosphere triggered mafic magma underplating, heating upper crust granites, and causing them partial melt.

Mesozoic multiple magmatism and porphyry–skarn Cu–polymetallic systems of the Yidun Terrane, Eastern Tethys: Implications for subduction- and transtension-related metallogeny

Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with relatively high Sr/Y features. The Mesozoic porphyry–skarn Cu–polymetallic systems of the Yidun Terrane hosted by the Late Triassic and Late Cretaceous intrusions provide chances to investigate the spatial and genetic relationships between subduction- and transtension-related magmatism and mineralization, and determine the petrogenesis and fertility of magmatic rocks. Zircon U–Pb ages indicate that the Late Triassic pre-ore quartz diorite porphyries and syn-ore quartz monzonite porphyries were emplaced at ~225 and ~215Ma, respectively. The Late Cretaceous syn-ore monzogranite porphyries were emplaced at 83–78Ma. Our new data, combined with previously published geochronological data, show the spatially overlapping distribution of the multiple Mesozoic porphyry systems in the Yidun Terrane. Although all the Late Triassic intrusive rocks share similar geochemical characteristics, the pre-ore quartz diorite porphyries have normal arc-related chemical features with low Sr/Y and (La/Yb)N ratios, high Y and Yb abundances, while the syn-ore quartz monzonite porphyries exhibit high Sr/Y and (La/Yb)N ratios, low Y and Yb abundances. All samples show similar Sr–Nd–Hf isotopic compositions [(87Sr/86Sr)i=0.7060–0.7117, εNd(t)=−6.7 to 0.0, zircon εHf(t)=−4.0 to +3.0], suggesting that they were probably derived from partial melting of juvenile lower crust. Trace-element patterns and partial melt modeling indicate that the quartz diorite porphyries were likely formed by partial melting of normal thick lower crust, while the causative quartz monzonite porphyries were probably formed by partial melting of eclogitized, thickened lower crust. We propose that pre-ore quartz diorite porphyries were probably generated earlier via the subduction of Garze–Litang oceanic crust, and syn-ore quartz monzonite porphyries were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. Thus, these high Sr/Y quartz monzonite porphyries host several economically important porphyry Cu deposits, such as Pualng, Xuejiping and Songnuo. However, the Late Cretaceous syn-ore monzogranite porphyries have lower εHf(t) values (−8.4 to −2.9) and εNd(t) values (−7.5 to −3.8) than the Late Triassic porphyries, indicating that the former mainly originated from ancient crustal materials. New dataset and previous studies suggest that the Late Cretaceous post-collisional transtension triggered the asthenospheric upwelling and the underplating of mafic magmas, induced the partial melting of garnet-bearing amphibolite and thus caused the emplacement of monzogranite porphyries and associated porphyry–skarn Cu–Mo deposits.

Characterization and origin of granites from the Luoyang Fe deposit, southwestern Fujian Province, South China

The Luoyang Fe deposit, located in southwestern Fujian Province (South China), is one of the Makeng-type skarn Fe deposits (i.e., skarn Fe deposits related to granite) related to the late Yanshanian (i.e., Early Cretaceous) granite. To better understand different mineral assemblages in different deposits, geochemistry and isotopic systematics of the granite magma source of the Luoyang Fe deposit were compared with those of other Makeng-type skarn Fe deposits in southwestern Fujian Province. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb dating of the nearby exposed Luoyang fine-grained granite and porphyritic granite yielded weighted mean 206Pb/238U ages of 139.6±1.4Ma and 137.2±2.3Ma, respectively. The Luoyang granite exhibits geochemical characteristics of highly fractionated I-type to A-type, consistent with the Pantian (PT) and Zhongjia (ZJ) granites in the Pantian and Zhongjia Fe deposits; whereas, the Dapai (DP) and Dayang–Juzhou (DJ) granites in the Dapai and Makeng Fe deposits show unfractionated I- and relatively typical A-type characteristics, respectively. It can be inferred from whole rock Sr–Nd–Pb and zircon Hf isotopes that the source magma of the Luoyang granite is coincident with other skarn Fe mineralization related granites of the Makeng-type deposits in southwestern Fujian Province. The magma was derived from partial melting of the Proterozoic metasedimentary Cathaysia basement rocks. Underplating of mafic magma in the depleted mantle and/or lower tholeiitic crust was also involved, which provided the heat source for partial melting. The change in tectonic environment and the different mafic sources involved caused significant differences in the Fe metallogenesis in southwestern Fujian Province.

Ages and petrogenesis of Jurassic and Cretaceous intrusive rocks in the Matsu Islands: Implications for lower crust modification beneath southeastern China

Major and trace element, whole-rock Sr-, Nd- and Hf-isotope, zircon U-Pb age and Hf-O isotope data are reported for the intrusive rocks from the Matsu Islands in the coastal area of southeastern (SE) China, in order to study the ages, sources and petrogenesis of these rocks and evolution of the lower crust. The rocks include gneissic granite, massive granite, brecciated granite and diabase. Secondary ion mass spectrometer (SIMS) zircon U-Pb dating reveals that the rocks in the Matsu Islands were emplaced at ∼160 Ma, ∼130 Ma and ∼94 Ma. The Jurassic granites (∼160 Ma) have high SiO2 (74.1–74.5 wt%) and K2O+Na2O (8.32–8.33 wt%) contents and high Rb/Sr ratios of 0.6–1.2 and (La/Yb)CN ratios of 12.6–19.4. Their relatively high initial 87Sr/86Sr ratios (0.7074–0.7101), variable and negative εNd(t) values (−9.2 to −5.4), and variable zircon εHf(t) (−17.0 to +5.2) and δ18O (4.7–8.1‰) values indicate they were mainly derived from an ancient lower crustal source, but with involvement of high εHf(t) and low δ18O materials. The Early Cretaceous diabase (∼130 Ma) has SiO2 content of 56.5 wt%, relatively high MgO concentration, low initial 87Sr/86Sr ratio and negative εNd(t) value, similar to geochemical features of other Cretaceous mafic rocks in the coastal area of SE China. Zircons from the diabase have high εHf(t) values (−5.5 to +0.2) and relatively low δ18O values of 4.2–5.0‰. These characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle, which have been metasomatised by altered oceanic crust-derived low-δ18O fluids. For the Cretaceous granitoids (∼130 Ma and 94 Ma), they have relatively low SiO2 (68.0–71.3 wt%) and K2O+Na2O (5.30–7.55 wt%) contents and low Rb/Sr ratios and (La/Yb)CN ratios of 5.8–7.1. They have low initial 87Sr/86Sr ratios (0.7071–0.7082), homogeneous εNd(t) (−4.3 to −4.5) and relatively high zircon εHf(t) values (−3.7 to +1.2) and low δ18O values (4.6–5.9‰). Their isotopic compositions are similar to those of the diabases in this study as well as other Cretaceous mafic rocks in the coastal area of SE China, suggesting that the sources of the Cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks. Whole-rock geochemical, Sr-Nd and zircon Hf-O isotopic compositions indicate that the Jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks, whereas the Cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle-derived magma. This distinction implies that the pre-existing ancient lower crust beneath the coastal area of SE China has been modified by large-scale mafic magma underplating. Therefore, underplating of mantle-derived mafic magma would result in modification of the pre-existing ancient lower crust and formation of the relatively juvenile lower crust.

Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt‐reintegration approach

ABSTRACTSuprasolidus continental crust is prone to loss and redistribution of anatectic melt to shallow crustal levels. These processes ultimately lead to differentiation of the continental crust. The majority of granulite facies rocks worldwide has experienced melt loss and the reintegration of melt is becoming an increasingly popular approach to reconstruct the prograde history of melt‐depleted rocks by means of phase equilibria modelling. It involves the stepwise down‐temperature reintegration of a certain amount of melt into the residual bulk composition along an inferredP–Tpath, and various ways of calculating and reintegrating melt compositions have been developed and applied. Here different melt‐reintegration approaches are tested using El Hoyazo granulitic enclaves (SE Spain), and Mt. Stafford residual migmatites (central Australia). Various sets ofP–Tpseudosections were constructed progressing step by step, to lower temperatures along the inferredP–Tpaths. Melt‐reintegration was done following one‐step and multi‐step procedures proposed in the literature. For El Hoyazo granulites, modelling was also performed reintegrating the measured melt inclusions and matrix glass compositions and considering the melt amounts inferred by mass–balance calculations. The overall topology of phase diagrams is pretty similar, suggesting that, in spite of the different methods adopted, reintegrating a certain amount of melt can be sufficient to reconstruct a plausible prograde history (i.e. melting conditions and reactions, and melt productivity) of residual migmatites and granulites. However, significant underestimations of melt productivity may occur and have to be taken into account when a melt‐reintegration approach is applied to highly residual (SiO2<55 wt%) rocks, or to rocks for which H2O retention from subsolidus conditions is high (such as in the case of rapid crustal melting triggered by mafic magma underplating).

Zircon U–Pb, molybdenite Re–Os geochronology and Sr–Nd–Pb–Hf–O–S isotopic constraints on the genesis of Relin Cu–Mo deposit in Zhongdian, Northwest Yunnan, China

The Relin Cu–Mo deposit is located in Zhongdian, southern portion of the Yidun Terrane, which was spatially and temporally associated with the Late Cretaceous intrusions. Orebodies at Relin are mainly hosted by monzogranite porphyry and hornfels with mineralization styles of dissemination and quartz-sulfide vein. LA–ICP–MS zircon U–Pb dating of four monzogranite porphyries yields 206Pb/238U ages varying from 83.9 to 79.0Ma, with four weighted mean 206Pb/238U ages of 80.6±0.3Ma, 82.8±0.3Ma, 80.9±0.5Ma and 81.6±0.5Ma. Re–Os isotope analyses of molybdenite separates from quartz-vein and altered granite ores yield model ages ranging from 84.3 to 81.3Ma. Geochemically, the ore-related monzogranite porphyries are metaluminous to slightly peraluminous, with an aluminum saturation index of 0.95–1.10. They are moderately rich in alkalis with K2O contents of 3.22–5.47wt% and Na2O of 2.97–3.93wt%, belonging to the high-K calc-alkaline to shoshonite series. Low MgO (0.68–1.85wt%), Cr (8.14–16.67ppm) and Ni (4.46–12.94ppm), negative zircon εHf(t) values (−9.1 to −2.7) and whole-rock εNd(t) values (−7.7 to −5.8), and relatively high zircon δ18O values (5.8–7.3‰) and radiogenic Pb isotopic compositions suggest that the Relin monzogranite porphyries were most likely generated by partial melting of the ancient lower crust. The calculated Hf isotope crustal model ages of 1.5–1.2Ga and Nd-isotope depleted mantle ages of 1.2–1.1Ga indicate that the ancient lower crust might belong to the Mesoproterozoic era. In addition, the Relin intrusions display fractionated rare earth element patterns with high (La/Yb)N and Sr/Y ratios, but low (Dy/Ho)N ratios, indicating a garnet-bearing amphibolite source. Breaking down of amphibole during partial melting of lower crust would generate hydrous and high fO2 magmas, which are favorable magmas to form the porphyry Cu–Mo deposit. Positive δ34S values (3.6–4.7‰) of sulfides point to a magmatic source of sulfur and ore-forming fluids in the Relin deposit. These results collectively show that the Relin deposit is a Late Cretaceous porphyry Cu–Mo deposit. Intraplate extension along NNW-SSE-trending sinistral faults and shear zones triggered asthenospheric upwelling and underplating of mafic magma, thus induced the partial melting of ancient lower crust. This explanation might have revealed the mechanisms of ore-forming and magma-emplacement for the porphyry deposits occurred under intra-continental extensional settings.

Petrogenesis of the Late Archean (∼2.5 Ga) Na- and K-rich granitoids in the Zhongtiao-Wangwu region and its tectonic significance for the crustal evolution of the North China Craton

Petrogenesis of coexisting Late Archean (∼2.5Ga) Na- and K-rich granitoids in the Zhongtiao-Wangwu region is significant for understanding the crustal evolution of the North China Craton (NCC). In this paper, we present new zircon U-Pb age, and whole-rock geochemical and isotope data on a ∼2.5Ga Na- and K-rich granitoid suite from the Zhongtiao-Wangwu region, southern NCC. The Na-rich granitoids (TTG) and K-rich granitoids (A-type granites) are zircon U-Pb dated to be 2.45Ga and 2.49Ga, respectively. Integrated geochemical data indicate that the TTG were formed by partial melting of the mafic lower crust, while the A-type granites were formed by partial melting of a TTG-like source in the shallower crust. The petrogenesis likely occurred under lithospheric delamination and the subsequent asthenospheric upwelling and mafic magma underplating. We proposed that the coeval occurrence of Na- and K-rich granitoids in the southern NCC is probably ascribed to the partial melting of continental crust at different depths, which could be an important way to induce the Archean crustal reworking and compositional changes at ∼2.5Ga.

Early Cretaceous Na-rich granitoids and their enclaves in the Tengchong Block, SW China: Magmatism in relation to subduction of the Bangong–Nujiang Tethys ocean

The Na-rich intermediate-to-felsic granitic rocks provide insights into the generation of magmas in subduction zones. This paper presents zircon LA-ICP-MS U–Pb ages as well as whole-rock geochemical, mineral chemical, and in situ zircon Hf isotopic data on Na-rich granitic rocks from the Tengchong Block, SW China. The granodiorites and associated mafic magmatic enclaves (MMEs) from the Menglian batholith yield zircon U–Pb ages of 116.1±0.8 to 117.8±0.6Ma and 117.7±0.7Ma, respectively. Both host granodiorites and enclaves show calc-alkaline and sodium-rich nature, enrichment in large-ion lithophile elements (LILEs), and variable depletion in zircon Hf isotopic compositions. Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine (An36–57) composition. The granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources. The quartz monzodiorite–granodiorites and associated MMEs from the Xiaotang-Mangdong batholith yield zircon U–Pb ages of 120.3±1.3 to 122.6±0.8Ma and 120.7±1.5Ma. These rocks are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions (−5.55 to +0.58). The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions. The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition. We infer that the quartz monzodiorite–granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas. The formation of the different types of Na-rich granitic rocks is correlated to the subduction of Bangong–Nujiang Tethyan ocean. A comparison with magmatism in the northern magmatic belt suggests that mantle-derived magmas played an important role in the genesis of Early Cretaceous intrusions from Tengchong to Lhasa Blocks, although crustal melting is the dominant contributor.

The Sveconorwegian Pegmatite Province – Thousands of Pegmatites Without Parental Granites

Abstract The Late-Proterozoic Sveconorwegian pegmatite province in southern Norway and southwest Sweden hosts seven rare-element pegmatite districts with more than 5000 rare-element pegmatites. Most of these pegmatites with Niobium-Yttrium-Fluorine (NYF) signature are not related to a parental granite, but instead occur in areas of high-grade metamorphism and are the result of migmatization and local melt collection. There are three groups of pegmatites: (1) rare-element pegmatites related to H P -H T high-grade metamorphism associated with the assembly of the Sveconorwegian orogen; (2) rare-element pegmatites related to post-orogenic extension with L P -H T granulites; and (3) rare-element pegmatites related to granite magmatism during post-orogenic extension. The pegmatite formation principally comprises four periods restricted to certain tectono-metamorphic domains: (I) 1094–1060 Ma (Bamble sector); (II) 1041–1030 Ma (Idefjord terrane); (III) 992–984 Ma (Idefjord terrane, Rogaland-Hardangervidda-Telemark sector); and (IV) 922–901 Ma (Rogaland-Hardangervidda-Telemark and Bamble sectors). The observed relationships between pegmatite formation and regional high-grade metamorphism reveal that the majority of Sveconorwegian pegmatites are formed by anatexis, either by crustal stacking during different stages of continental/terrane collision (H P metamorphism) (periods I to III), or by mafic magma underplating (H T metamorphism) during orogenic extension (period IV). In several provinces that have been affected both by early H P metamorphism during continental collision and by late H T metamorphism during crustal extension, there may occur several generations of pegmatites that show mineralogical and geochemical affinity, even though they formed during several different periods. In addition, the results imply that the majority of Sveconorwegian NYF pegmatites are not necessarily formed in an anorogenic setting in relation to A-type magmatism, but in compressional or extensional orogenic settings unrelated to pluton-scale magmatism. In light of this, the genetic criteria of the pegmatite family classification scheme [NYF versus Lithium-Cesium-Tantalum (LCT)] will have to be re-evaluated.

Two mineralization events in the Baiyinnuoer Zn-Pb deposit in Inner Mongolia, China: Evidence from field observations, S-Pb isotopic compositions and U-Pb zircon ages

The Xing–Mong Orogenic Belt (XMOB) is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and has experienced multiple tectonic events. The Baiyinnuoer Pb-Zn deposit may be a rare case that documents two periods of mineralization in the tectonically complex XMOB. There are two types of Pb-Zn mineralization in the deposit: (1) skarn-type ore, hosted by the skarn in the contact zone between marble and granodiorite and within the marble and (2) vein-type ore, hosted by crystal tuff and feldspar porphyry. This study revealed that the host rocks, mineral assemblages, mineralization occurrences, S-Pb isotopes, and ages between the two types of ore are notably different. Zircon U-Pb dating indicates that the granodiorite was emplaced in the Early Triassic (244±1 to 242±1Ma), the crystal tuff was deposited in the Early Cretaceous (140±1 to 136±1Ma), and the feldspar porphyry was intruded in the Early Cretaceous (138±2 to 136±2Ma). The first skarn mineralization occurred at ∼240Ma and the second vein-type Pb-Zn mineralization took place between 136 and 129Ma. Thus the Triassic orebodies were overprinted by Early Cretaceous mineralization.The sphalerite and galena from the skarn mineralization have higher δ34S values (−4.7 to +0.3‰) than the sphalerite, galena and aresenopyrite from the vein-type mineralization (−7.5 to −4.2‰), indicating different sulfur sources or ore-forming processes for the two types of mineralization. The Pb isotopic compositions of the two types of ore are very similar, suggesting similar lead sources. Geochemistry and Nd-Pb-Hf isotopic systematics of the igneous rocks in the region show that the Triassic granodiorite was generated from hybridization of mafic and felsic magmas due to strong crust-mantle interaction under the collisional setting that resulted following the closure of the Paleo-Asian Ocean and the collision of North China and Siberian cratons at the end of the Permian; while the Cretaceous igneous rocks at Baiyinnuoer originated from the partial melting of a juvenile lower crust with minor input from the crust caused by the underplating of mafic magma in an extensional setting.