Arc-like Trace Element Distribution Patterns Research Articles

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.

Trace elements and Li isotopes distinguish between liquid and solid phases contributing to mafic arc magmas

It is generally accepted that metasomatism of the mantle wedge peridotite by subducting slab-derived fluids plays a key role in dictating the composition of mafic arc magmas. However, this fluid metasomatism model has been challenged recently by the mélange diapirism model. In order to distinguish between liquid and solid phases that would exclusively contribute to mafic arc magmas, we have carried out an integrated study of whole-rock and mineral geochemistry for two types of contemporaneous mafic igneous rocks (gabbroic diorites and diorite dikes) from an accretionary orogen. The results indicate primary contributions from liquid phases composed of not only slab-derived aqueous solution but also seafloor sediment-derived hydrous melt. Although these mafic rocks all exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li and radiogenic Sr–Nd–Hf isotope compositions as well as other geochemical variables. In general, the gabbroic diorites are characterized by enrichment in fluid-mobile elements (e.g., Rb, Ba, K, Pb, and Sr), high δ7Li and δ18O values, and depletion in Sr–Nd–Hf isotope compositions. By contrast, the diorite dikes exhibit enrichment in melt-mobile elements (e.g., LILE, LREE and Th), relatively low δ7Li values, and slight enrichment in Sr–Nd–Hf isotope compositions. There are also systematic differences and pertinent variations between Li isotope compositions and slab-derived fluid indicators such as whole-rock Rb/Nd, Sr/Nd, Nb/U, Th/La, Th/Nd and Th/Yb ratios for the two types of mafic igneous rocks. These indicate that the mantle sources of these mafic rocks would possess geochemical signatures that were acquired through chemical metasomatism by slab-derived fluids rather than physical mixing with mélange diapirs. Furthermore, the geochemical signatures allow for distinction between the oceanic slab-derived aqueous solution and the seafloor sediment-derived hydrous melt. This qualitative interpretation is quantitatively confirmed by model calculations of trace elements and Sr–Nd–Li isotopes in the two types of mafic igneous rocks. Therefore, the target mafic igneous rocks provide the geochemical evidence for the formation of their mantle sources through chemical metasomatism by incorporating the two types of metasomatic agent into the mantle wedge. This demonstrates a strong preference for the fluid metasomatism model over the mélange diapirism model for the origin of mafic arc magmas.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Heterogeneous orogenic lithospheric mantle beneath the North Qaidam orogen: Geochemical evidence from syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U Pb ages and Hf O isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon U Pb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn -exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ 18 O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn -exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn -exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn -exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone. • Syn-exhumation/post-collisional mafic magmatic rocks in the North Qiadam orogen. • They were derived from a heterogeneous orogenic lithospheric mantle. • The mantle source was metasomatized by melts from the subducted continental crust. • These rocks record crust-mantle interaction in the continental subduction channel.

The effects of source composition and melting conditions on the composition of syn-exhumation granites in collisional orogen

Partial melting of the deeply subducted continental crust during exhumation has been widely recognized recently in collisional orogens. However, it is little known about the effects of source composition and melting conditions on the composition of syn -exhumation granitic rocks. This is investigated by studying zircon and titanite U Pb geochronology and whole-rock geochemistry for syn -exhumation granites from the Weihai area in the Sulu orogen. The U Pb dating of relict zircon cores yields concordant Neoproterozoic and Late Triassic ages, respectively, for the meta-igneous protolith and granulite-facies metamorphism. Magmatic zircon rims give U Pb ages of 207 ± 1 to 214 ± 2 Ma for the formation of Weihai granites, in agreement with magmatic titanite ages of 209 ± 6 to 217 ± 5 Ma in the Late Triassic. The compositional zonation of titanite shows decreases of Nb/Ta and (La/Yb) N from core to rim, indicating the geochemical changes during granitic magmatism. In general, these granites show arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with an affinity to deeply subducted crustal slices from the South China Block. Compared with previously reported syn -exhumation granites from Lanshan in the southern Sulu orogen, the Weihai granites in the northeastern Sulu orogen exhibit different Sr-Nd-O isotope compositions. The Weihai granites have relatively low SiO 2 and Na 2 O contents but high Al 2 O 3 and K 2 O contents with high A/CNK and K/Na ratios. They contain high Ba and Sr but low Y with high Sr/Y and (La/Yb) N ratios. Both zircon and whole-rock of the Weihai granites exhibit no significant negative Eu anomalies. Based on the differences in geochemical features and the spatial-temporal distributions, it is inferred that the syn -exhumation granites in the Weihai area are derived from decompressional melting of the lower crustal slice at greater depth, whereas those in the Lanshan area are derived from decompressional melting of the upper crustal slice at relatively shallower depth. These differences indicate that both source compositions and melting conditions have exerted significant influences on the geochemical composition of syn -exhumation granites in the collisional orogen. This has bearing on the crust-mantle interaction in continental subduction channel through metasomatic reaction of the mantle wedge with these granitic melts produced during the exhumation. • The Weihai granites in the northern Sulu orogen indicate syn -exhumation magmatism. • The Weihai granites are characterized by relatively high Sr/Y and (La/Yb) N ratios. • Syn-exhumation granites in the Sulu orogen are distinct from south to north. • Differences in source compositions and melting conditions caused the compositional diversity of granites.

Post-collisional reworking of subducted continental crust: Insights from late Paleozoic granites in the North Qaidam orogen, northeastern Tibet

Post-collisional granites are widely distributed in continental collision orogenic belts, and deciphering their petrogenesis has great bearings for understanding the deep crustal compositional and tectono-thermal evolution of collisional orogens. Here we carried out a combined study of whole-rock major-trace elements and Sr–Nd–Hf isotopes, and zircon UPb ages and LuHf isotopes for late Paleozoic granites from the North Qaidam orogen, northeastern Tibet. The results indicate that they were derived from partial melting of subducted continental crust at high thermal gradients due to post-collisional rifting orogeny. Zircon UPb dating of the granites yields weighted mean 206Pb/238U ages between 377 ± 3 Ma and 384 ± 5 Ma, which postdate the ultrahigh-pressure (UHP) metamorphic ages (ca. 438–420 Ma) of deeply subducted continental crust in the North Qaidam orogen. Therefore, they are products of post-collisional magmatism. Most relict zircons in the granites have concordant Neoproterozoic (793–972 Ma) and early Paleozoic (405–442 Ma) UPb ages, generally consistent with the protolith and metamorphic ages of the UHP metaigneous rocks in the orogen, respectively. The granites are commonly calc-alkaline to high-K calc-alkaline with total alkali contents of 6.89–8.56 wt%, and weakly to moderately peraluminous with A/CNK values of 1.00–1.18 [A/CNK = Al2O3/(CaO + Na2O + K2O), in molar]. They exhibit arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with whole-rock (87Sr/86Sr)i ratios of 0.7074–0.7104, ɛNd(t) values of −9.0 to −4.4 and ɛHf(t) values of −4.2 to +2.9, and syn-magmatic zircon ɛHf(t) values of −7.6 to +7.7 (except one analysis of −15.0). These granites mostly have comparable SrNd isotope compositions to the UHP orthogneiss and continental eclogite in this collisional orogen, indicating their derivation from the subducted continental crust. Using the average compositions of the orthogneiss and continental eclogite in the orogen as the starting materials, phase equilibrium modelling results suggest that the whole-rock major elements of the post-collisional granites can be explained by the mixed melts mainly derived from these two lithologies, which were partially melted at high thermobaric ratios of 1156–1867 °C/GPa associated with different water contents (1–5 wt% for continental eclogite, 1–3 wt% for orthogneiss). Therefore, this study provides integrated geochemical constraints on the source rocks of the post-collisional granites and physicochemical conditions for their genesis.

Magma differentiation and recharge in the petrogenesis of early paleozoic mafic intrusives in the Qilian orogen, northwestern China

An integrated study of zircon UPb ages and LuHf isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as amphibole and plagioclase major-trace elements is presented for early Paleozoic mafic intrusives from the Heishishan area in the Qilian orogen, northwestern China. The results provide not only constraints on the origin of their mantle sources but also insights into the effects of magma differeniation and recharge on their compositional variation. Zircon UPb dating yields consistent ages of 449 ± 2 Ma and 449 ± 3 Ma, respectively, for hornblende gabbro and diorite. These mafic intrusives show arc-like trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions with whole-rock (87Sr/86Sr)i ratios of 0.7040 to 0.7063, εNd(t) values of −2.3 to −0.3 and εHf(t) values of 1.6 to 4.9, and zircon εHf(t) values of 0.2 to 5.9. These geochemical features indicate their origination from mantle sources that were generated through metasomatism by subducting Proto-Tethyan oceanic crust-derived fluids. Some samples exhibit high Th/Nb and (La/Yb)N ratios, suggesting that the metasomatic agents would be composed of seafloor sediment-derived hydrous melts and oceanic igneous crust-derived aqueous solutions. The consequent metasomatites in the mantle wedge would be lithochemically fertile and geochemically enriched and therefore are susceptible to partial melting for mafic magmatism. Amphibole and plagioclase in the studied igneous rocks show complex textural and compositional zonings, recording four stages of mineral crystallization during magma evolution. Combining the whole-rock and mineral geochemical characteristics together, we suggest that different batches of mafic melts would be produced by partial melting of the metasomatic mantle sources, experiencing variable degrees of magma differentiation and recharge prior to emplacement at the subsurface. Therefore, in addition to the nature of crust-mantle interaction at mantle depths, the processes of magma evolution and recharge at crustal depths would have also played an important role in the petrogenesis of arc-type mafic igneous rocks above the oceanic subduction zone.

New identification and significance of Early Cretaceous mafic rocks in the interior South China Block

Early Cretaceous mafic rocks are first reported in the northern Guangxi region from the western Qin-Hang belt in the interior South China Block. A systematic investigation of zircon U–Pb dating, whole-rock geochemistry, Sm–Nd isotopes and zircon Hf–O isotopes for these mafic rocks reveals their petrogenesis and the mantle composition as well as a new window to reconstruct lithospheric evolution in interior South China Block during Late Mesozoic. Zircon U–Pb dating yielded ages of 131 ± 2 Ma to 136 ± 2 Ma for diabase and gabbro from Baotan area, indicating the first data for Early Cretaceous mafic magmatism in the western Qing-Hang belt. These mafic rocks show calc-alkaline compositions, arc-like trace element distribution patterns, low zircon εHf(t) of − 9.45 to − 6.17 and high δ18O values of + 5.72 to + 8.09‰, as well as low whole-rock εNd(t) values of − 14.27 to − 9.53. These data suggest that the studied mafic rocks are derived from an ancient lithospheric mantle source that was metasomatized during Neoproterozoic subduction. Thus, the occurrence of these mafic rocks indicates a reactivation of Neoproterozoic subducted materials during an extension setting at Late Mesozoic in the western Qin-Hang belt, an old suture zone that amalgamates the Yangtze and Cathaysia blocks.

Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.

Bimodal volcanic rocks in the northeastern margin of the Yangtze Block: Response to breakup of Rodinia supercontinent

Mid-Neoproterozoic protoliths of high–ultrahigh-pressure metamorphic rocks in the northeastern margin of the Yangtze Block preserve one of the best examples of 18O-depleted magmatic provinces worldwide, which is the significant record for lithospheric extension associated with continental rifting. In this contribution, we report the depleted 18O signature of felsic meta-volcanic rocks from the Zhangbaling Uplift (ZBLU), a segment of the Tan–Lu fault zone, which is located between the Dabie and Sulu orogenic belts. The low-δ18O signature of these rocks provides crucial information for understanding their origin and tectonic setting. Zircons yield UPb protolith ages of 771 ± 5 Ma to 761 ± 5 Ma and show a narrow range in δ18O values of 1.97‰ to 3.11‰. The typical syn-magmatic characteristics of these zircons indicate that they record inheritance from low-δ18O magmas during the mid-Neoproterozoic, which suggests the occurrence of high-temperature surface-water–rock interaction in a continental rift setting. Quartz keratophyres in the ZBLU exhibit A-type granite geochemical compositions with high contents of high-field-strength elements (Zr + Nb + Ce + Y > 350 ppm) and a zircon saturation temperature of 829 °C. The zircon grains have slightly depleted εHf(t) values of −3.4 to +1.9, which suggest Mesoproterozoic juvenile crustal formation in response to the amalgamation of the Yangtze Block with the Rodinia supercontinent. A few coeval spilites (758 Ma) crop out sporadically in the ZBLU. Their geochemical characteristics include high SiO2 contents (51.06–57.03 wt%), low Mg# values (42–53), and arc-like trace-element distribution patterns. The variable zircon εHf(t) values for the spilite samples can be divided into two groups: negative εHf(761 Ma) values of −16.4 to −11.1, and positive εHf(744 Ma) values of +5.9 to +8.5, which supports source mixing between enriched mantle and residual oceanic crustal material. In conclusion, the ZBLU spilite–keratophyre sequence is composed of a typical bimodal rock association, providing new petrological evidence for the origin of low-δ18O magmas and for a continental rift setting related to the separation of the Yangtze Block from the Rodinia supercontinent at ca. 771 Ma.

Geochemical Distinction Between Altered Oceanic Basalt- and Seafloor Sediment-Derived Fluids in the Mantle Source of Mafic Igneous Rocks in Southwestern Tianshan, Western China

AbstractThe role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr–Nd–Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U–Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd–Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0·7039– 0·7057. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5·2–7·6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0·8 to 3·5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr–Nd–Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones.

Identification of Jurassic mafic arc magmatism in the eastern North China Craton: Geochemical evidence for westward subduction of the Paleo-Pacific slab

AbstractSubduction of the Paleo-Pacific slab beneath the North China Craton (NCC) has exerted a strong influence on the Mesozoic destruction of the craton. However, no Andean-type arc magmatism has been reliably identified in the eastern NCC. Here we report the occurrence of Jurassic arc-like lamprophyres in the Liaodong Peninsula, providing a snapshot of the Paleo-Pacific slab subduction beneath the NCC in the early Mesozoic. Zircon U-Pb dating of the lamprophyres yields consistent ages of 158–155 Ma for magma crystallization. These lamprophyres all exhibit typical arc-like trace element distribution patterns, but show a series differences in their radiogenic isotope compositions and the other geochemical variables. Type 1 lamprophyres exhibit weakly enriched Sr-Nd-Hf isotopes with (87Sr/86Sr)i ratios of 0.7075–0.7085, εNd(t) values of −3.9 to −1.3 and εHf(t) values of −5.4 to −0.3, whereas Type 2 lamprophyres exhibit moderately enriched radiogenic isotopes with (87Sr/86Sr)i ratios of 0.7096–0.7117, εNd(t) values of −12.2 to −7.6 and εHf(t) values of −12.8 to −4.7. There are also systematic differences in zircon Hf isotopes and whole-rock Ba/Th, Ba/La, Sr/Nd, Th/Nd, Th/Yb, and La/Sm ratios for the two types of lamprophyre. Taken together, these similarities and differences can be accounted for by metasomatic reaction of the cratonic mantle wedge with two properties of liquid phase derived from subducting Paleo-Pacific slab. One is aqueous solutions from the subducting basaltic oceanic crust, and the other is hydrous melts from the subducting terrigenous. The two properties of subduction zone fluids were incorporated in different proportions into the mantle sources of these lamprophyres. Accordingly, the lamprophyres were derived from the metasomatic mantle sources. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer at the slab-mantle interface in a paleo-oceanic subduction zone. Therefore, the Jurassic lamprophyres in the eastern NCC provide the geochemical evidence for the crust-mantle interaction during the Paleo-Pacific slab subduction beneath eastern Asia in the early Mesozoic, when the chemical metasomatism by the slab-derived fluids would have weakened the cratonic mantle for its thinning and destruction in the Early Cretaceous.

Syn-exhumation magmatism in an active continental margin above a continental subduction zone: Evidence from Late Triassic mafic igneous rocks in the southeastern North China Block

AbstractSyn-subduction arc magmatism is absent above continental subduction zones, providing important constraints on the nature of petrogenetic processes during continental collision. Nevertheless, we have identified syn-exhumation mafic igneous rocks in an active continental margin above a continental subduction zone, where the South China Block (SCB) was deeply subducted beneath the North China Block (NCB) for collisional orogeny in the Triassic. These mafic igneous rocks occur in the southeastern margin of the NCB, showing consistent Late Triassic zircon U-Pb ages of ca. 219–218 Ma, coeval with exhumation of the deeply subducted continental crust. These rocks are categorized into two series of sub-alkaline and alkaline, all exhibiting arc-like trace element distribution patterns, highly enriched radiogenic Sr-Nd-Hf and high zircon O isotope compositions. In particular, they exhibit two-stage whole-rock Nd and Hf model ages and zircon Hf model ages of Paleoproterozoic, which are comparable to those of ultrahigh-pressure metamorphic rocks with the SCB affinity in the Dabie-Sulu orogenic belt. Such geochemical features indicate that these mafic igneous rocks were derived from partial melting of ultramafic metasomatites generated by reaction of felsic melts from the subducted SCB with the mantle wedge peridotite beneath the NCB. The geochemical differences in element and isotope compositions between the two series igneous rocks can be mainly ascribed to different proportions of the crustal component in the metasomatites, which is verified by quantitative modellings of the geochemical transfer in the continental subduction zone. The systematic variations in some geochemical variables such as Fe/Mn, Zn/Fe, and Nb/Ta ratios indicate pyroxenite-rich and hornblendite-rich lithologies, respectively, for the sub-alkaline and alkaline series igneous rocks. With the tectonic extension for exhumation of the deeply subducted continental crust in the Late Triassic, the fertile and enriched metasomatites in the mantle wedge underwent partial melting for the syn-exhumation mafic magmatism in the southeastern NCB. Therefore, the mafic igneous rocks in the active continental margin not only record the crust-mantle interaction in the continental subduction zone, but also witness the generation of syn-exhumation magmatism in the late stage of continental collision.

Lithium isotopic compositions of post-collisional mafic–ultramafic rocks from Dabieshan, China: Implications for recycling of deeply subducted continental crust

Lithium (Li) isotopes are potential tracers of processes related to crust–mantle interaction and recycled crustal materials. Previous studies have reported that Li isotopic compositions of island arc lavas are similar to those of fresh mid-ocean-ridge basalts (MORBs), although the former are expected to record high slab-derived δ7Li values caused by fluid metasomatism. However, it is unclear whether Li isotopic signatures of subducted continental crust were transported to mantle depths and preserved there. For clarification, we determined whole-rock major and trace element, and Li isotopic compositions of post-collisional mafic–ultramafic rocks from northern Dabieshan, China. The analyzed samples have high MgO (up to 20.67 wt%) and low SiO2 (41.06–56.20 wt%) contents, and arc-like trace element distribution patterns, suggesting derivation by partial melting of fertile mantle sources. The samples have δ7Li values of −2.9 to +7.2‰ (average = +2.7‰, n = 42) and Li concentrations of 2.50 to 28.1 μg/g (average = 9.20 μg/g), both of which are beyond the relatively limited ranges of mantle values (δ7Li = +2.9 to +4.3‰, average = +3.5 ± 1.0‰, and Li = 1.20 ± 0.10 μg/g). Excluding possible effects of wall-rock assimilation and contamination, magmatic crystallization, and Li isotopic fractionation during subduction–exhumation processes, the observed Li isotopic heterogeneity is most likely related to the recycling of crustal materials. Based on a comparison with Li isotopic compositions of ultrahigh-pressure (UHP) metamorphic rocks from the Dabie–Sulu orogenic belt and terrestrial sediments, we suggest that the recycled continental crustal materials likely comprised gneisses, sediments, eclogites, and/or minor carbonate rocks (i.e., marbles). Our study demonstrates that Li isotopes may provide a novel means of tracing deeply subducted continental materials.

Petrogenesis and geodynamic setting of Early Cretaceous felsic rocks in the Gan-Hang Belt, Southeast China: Constraints from geochronology and geochemistry of the tuffs and trachyandesitic rocks in Shengyuan volcanic Basin

The Late Mesozoic geology of the Gan-Hang Belt is characterized by extensive magmatism forming a belt of volcanic-intrusive complex. The geochronology, petrogenesis, and geodynamic setting of the Late Mesozoic magmatic rocks in the Gan-Hang Belt are still controversial. The Shengyuan volcanic Basin is located in the NW region of the belt and mainly contains crystal tuff, welded tuff, and trachyandesitic rocks. We integrate geochronological and geochemical data for these tuffs and trachyandesitic rocks, to explore the origin of these rocks and improve our understanding of the Late Mesozoic magmatic and tectonic evolution of the region. Zircon U-Pb dating shows that these samples were formed in the Early Cretaceous (135–137Ma). All the tuffs have a pronounced A2-type geochemical signature and their chemical compositions are controlled by crystal fractionation. All trachyandesitic rocks exhibit high K2O contents and were attributed to the shoshonite series; they are characterized by arc-like trace element distribution patterns, with significant enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field-strength elements (HFSE). Moreover, all the tuffs and trachyandesitic rocks were characterized by negative whole rock εNd(t) and zircon εHf(t). An integrated interpretation of all these geochemical data leads to the conclusion that the Shengyuan trachyandesitic rocks were primarily derived from mantle materials and oceanic crust-derived melt. The Shengyuan tuffs were formed by the partial melting of the Proterozoic orthometamorphic and parametamorphic rocks. Our studies together with previous published data suggest that the Early Cretaceous A-type felsic rocks, with ages between 138Ma and 122Ma, occurred along the Gan-Hang Belt, indicating an important Late Mesozoic extensional event in the Belt. This event represents a back-arc tectonic setting due to the rollback of the Paleo-Pacific plate.

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACTWe report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.

Geochemical constraints on the nature of magma sources for Triassic granitoids from South Qinling in central China

A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd isotopes as well as mineral chemistry and O isotopes was carried out for Triassic granitoids from the South Qinling orogen in central China. Model calculations were also performed to examine the trace element fractionation during partial melting of crustal rocks. The results provide insights into the nature of magma sources for these granitoids. LA-ICPMS zircon U-Pb dating yields concordant ages of 208±2 to 216±3Ma for these granitoids from the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) plutons, and no relict zircon cores are identified by the CL imaging and U-Pb dating. The SHW and CP granitoids contain hornblende and are metaluminous with A/CNK ratios of 0.84 to 0.93. They exhibit relatively low SiO2 contents (62.88–69.04wt.%) but high contents of FeOT, MgO and TiO2, and slightly to negligibly negative Eu anomalies (δEu=0.79–0.89). Zircons from them show mantle-like δ18O values of 4.71 to 5.72‰. In contrast, the ZS granites contain no hornblende and are metaluminous to weakly peraluminous with A/CNK ratios of 0.99 to 1.03. They show relatively high SiO2 contents (69.32–75.94wt.%) but low FeOT, MgO and TiO2 contents, and moderate negative Eu anomalies (δEu=0.63–0.81). They have slightly low zircon δ18O values of 4.60 to 4.83‰. All of these granitoids show arc-like trace element distribution patterns with enrichment in LREE and LILE (e.g., Rb, K and Pb) but depletion in HFSE (e.g., Nb, Ta and Ti). Geochemical comparison and modeling indicate that these granitoids are different from adakitic rocks originating from the thickened lower continental crust. Compared with the composition of felsic melts produced by petrological experiments of various lithologies, it appears that these granitoids are derived from dehydration melting of metabasaltic sources at normal lower crustal depths, and experienced varying degrees of fractional crystallization. The SHW and CP granitoids were crystallized under the conditions of variably low pressures of 0.7–2.3kbar, temperatures of 628–766°C and higher fO2 than the ZS granites. On the other hand, these granitoids show moderate whole-rock initial 87Sr/86Sr ratios of 0.7045 to 0.7055 and slightly negative εNd(t) values of −4.0 to −1.5 as well as slightly negative to positive zircon εHf(t) values of −1.3 to 3.2. They have two-stage Nd model ages of 1.05–1.38Ga and Hf model ages of 1.04–1.39Ga. These whole-rock εNd(t) and zircon εHf(t) values are comparable with those for Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in South Qinling but inconsistent with those for Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the northern margin of the Yangtze craton. Together with their non-adakitic features, it is suggested that they were derived from partial melting of the mafic lower crust in South Qinling rather than the subducted Yangtze continental crust beneath South Qinling. The continental collision between the North China Block and South China Block in the Late Triassic would have brought about reworking of the mafic lower crust for the granitoid magmatism in the Qinling orogen.

Partial melting of the orogenic lower crust: Geochemical insights from post-collisional alkaline volcanics in the Dabie orogen

Alkaline igneous rocks are a minor component in common associations of felsic to mafic rocks. Although they are usually regarded as the product of mantle-derived magmatism, they are possibly produced by partial melting of the overthickened orogenic crust. This is illustrated by post-collisional alkaline volcanics from the Dabie orogen in China. A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes, and zircon U-Pb ages and Hf-O isotopes for these volcanics provides geochemical constraints on the nature of their magma sources and thus insights into reworking of the lower continental crust in a collisional orogen. SIMS zircon U-Pb dating yields consistent Early Cretaceous ages of 136±2Ma and 133±2Ma for magma emplacement, contemporaneous with early-stage adakitic rocks in the Dabie orogen. Relict zircons record Neoproterozoic and Triassic U-Pb ages, respectively, in agreement with ages for protolith formation and ultrahigh-pressure metamorphism in the orogen. The alkaline volcanics are composed of phonolite and trachyte, with variable contents of SiO2 (55.94–64.10wt%), MgO (0.16–0.95wt%), Al2O3 (16.80–21.32wt%), K2O (4.65–14.55wt%) and Na2O+K2O (11.51–15.01wt%). They show arc-like trace element distribution patterns, with enrichment in LILE and LREE but depletion in HFSE. They exhibit enriched whole-rock Sr-Nd-Hf isotope compositions, with generally high (87Sr/86Sr)i ratios of 0.7074 to 0.7104, but negative εNd(t) values of −16.3 to −15.1 and negative εHf(t) values of −18.4 to −16.3. Zircon Hf-O isotope analyses yield less variable εHf(t) values of −27.8 to −23.0 and δ18O values of 4.9 to 5.9‰ for magmatic zircons, but more variable εHf(t) values of −32.7 to −6.5 and δ18O values of 3.0 to 10.2‰ for Neoproterozoic and Triassic relict zircons. Such geochemical features indicate that these alkaline volcanics were derived from partial melting of the subducted continental crust of the South China Block in the Dabie orogen. The magma sources would be dominated by the mafic igneous rocks of middle Neoproterozoic age. Additional contributions were from supracrustal rocks with high δ18O and variable εHf(t) values, which were incorporated into the overthickened orogenic crust during the Triassic continental collision. The orogenic crust underwent low-degree partial melting during its thinning at the post-collisional stage, giving rise to the alkaline volcanics in the Early Cretaceous.

Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

An integrated study of zircon U–Pb ages, Hf–O isotopes, whole-rock major-trace elements and Sr–Nd–Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle–Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1–3.7 wt.% and low Mg# values of 1.6–51.2, high K2O contents of 2.0–9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr–Nd–Hf–Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050–0.7082, negative εNd(t) values of −8.1 to −1.8, and negative εHf(t) values of −6.7 to 0.4. Zircon U–Pb dating yields consistent ages of 128±2 to 133±2Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7–7.8‰ and variable εHf(t) values of −5.8 to 0.7. All andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle–Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical composition of andesites is primarily governed by their mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesites but they are not the governing factors for their trace element and radiogenic isotope compositions.

Termination time of peak decratonization in North China: Geochemical evidence from mafic igneous rocks

Geophysical and petrological data indicate destruction of the cratonic lithosphere in North China in the Mesozoic, resulting in replacement of the ancient subcontinental lithospheric mantle (SCLM) by the juvenile SCLM. However, it remains to be answered when the craton destruction would have been terminated in the Mesozoic. This question is resolved by studying the two types of mafic igneous rocks with contrasting geochemical compositions from North China. The first type of mafic igneous rock shows arc-like trace element distribution patterns and enriched radiogenic Sr–Nd isotope compositions, with emplacement ages spanning from the Triassic to Early Cretaceous. The mafic magmatism is absent in a period from ~200Ma to ~135Ma, recording the thinning of cratonic lithosphere due to the westward flat subduction of the Paleo-Pacific slab beneath the North China Craton. In contrast, the second type of mafic igneous rocks exhibits oceanic island basalts (OIB)-like trace element distribution patterns and relatively depleted radiogenic Sr–Nd isotope compositions, with emplacement ages spanning from the Early Cretaceous to Cenozoic. Zircon U–Pb dating yields an age of ~121Ma for the geochemical transformation between the two types of mafic igneous rocks. This age marks a dramatic demarcation in the composition of their mantle sources. As such, the nature of mantle lithosphere in North China was changed from the ancient SCLM to the juvenile SCLM at ~121Ma. Thus, this age not only signifies the tectonic transition from the enriched mantle to the depleted mantle in the Early Cretaceous, but also dates the termination of peak decratonization in North China. Therefore, the craton destruction in the Early Cretaceous is temporally and spatially associated with the dramatic changes in the geochemical composition of mantle lithosphere.