Qing Qian

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities.

Partial melting of lower crust at 10-15 kbar: constraints on adakite and TTG formation

The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15xa0kbar and 800–1,050xa0°C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclasexa0+xa0amphibolexa0±xa0garnet) at 10–15xa0kbar and 800xa0°C, garnet granulite (plagioclasexa0+xa0amphibolexa0+xa0garnetxa0+xa0clinopyroxenexa0+xa0orthopyroxene) at 12.5xa0kbar and 900xa0°C, two-pyroxene granulite (plagioclasexa0+xa0clinopyroxenexa0+xa0orthopyroxenexa0±xa0amphibole) at 10xa0kbar and 900xa0°C and 10–12.5xa0kbar and 1,000xa0°C, garnet pyroxenite (garnetxa0+xa0clinopyroxenexa0±xa0amphibole) at 13.5–15xa0kbar and 900–1,000xa0°C, and pyroxenite (clinopyroxenexa0+xa0orthopyroxene) at 15xa0kbar and 1,050xa0°C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20xa0wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20xa0wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40xa0wt% melting of lower crust composition at 10–12.5xa0kbar and 800–900xa0°C and 15xa0kbar and 800xa0°C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050xa0°C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5xa0kbar and 1,000xa0°C and 15xa0kbar and 1,050xa0°C have elevated HREE, whereas partial melts at 13.5–15xa0kbar and 900–1,000xa0°C in equilibrium with >20xa0wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950xa0°C and 10–12.5xa0kbar (corresponding to a depth of 30–40xa0km), whereas a depth of >45–50xa0km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

Geochemistry of the Early Paleozoic Baiyin Volcanic Rocks (NW China): Implications for the Tectonic Evolution of the North Qilian Orogenic Belt

The Qilian Mountains in NW China comprise the North Qilian Orogenic Belt, Central Qilian Block, and South Qilian Orogenic Belt. The North Qilian Orogenic Belt consists of the Northern and Southern terranes separated by a volcanic rock belt. This belt is composed mainly of felsic and mafic volcanic rocks. Volcanic rocks in the Baiyin area of the eastern part of the belt include rhyolites, rhyodacite, andesitic basalts, and basalts. New zircon U‐Pb isotopic data yield a crystallization age of ca. 445 Ma for the rhyolite, 30 m.yr. younger than the associated basalts. The mafic volcanic rocks are relatively enriched in Th, Sr, and light rare earth element with (La/Yb)N ratios ranging from 4.2 to 5.6 and LaN ranging from 40 to 49, and depleted in high field strength elements, with high Th/Nb ratios (0.9–1.3). These features together with their ϵNd(T) values (−1.4 to +3.1) are consistent with a subduction‐related origin, most likely in a mature island arc or an arc built on thin continental crust in an active continental margin. The felsic volcanic rocks show a calc‐alkaline affinity and a strong suprasubduction zone signature with negative Nb, Sr, and Ti anomalies and relatively high Th/Nb ratios (0.8–1.6). They have significantly high ϵNd(T) values (+4.4 to +7.7) relative to the mafic volcanic rocks. Such radiogenic Nd isotopic compositions rule out a crustal origin and indicate the derivation from a depleted mantle source in a volcanic arc environment. Therefore, the geochemistry of the mafic and felsic volcanic rocks demonstrates an Ordovician volcanic arc above a northward subduction zone. The northward drifting of the Central Qilian Block eventually resulted in the amalgamation of the Northern and Southern terranes to form the North Qilian Orogenic Belt in the Early Paleozoic.

Comparative diffusion coefficients of major and trace elements in olivine at ∼950 °C from a xenocryst included in dioritic magma

which is a prerequisite for studying those trace elements for which substitution into a crystal requires some means of charge balance. In addition, the concentrations of the trace elements at the crystal-melt interface yield partition coeffi cients that may be checked against those obtained from equilibrium partitioning experiments, to test whether the concentrations of the diffusing elements are compatible with equilibrium substitution mechanisms. The method’s disadvantage is that relatively long diffusion profi les (>100 µm) are needed, which has prevented it being used in laboratory experiments at low temperatures where only short profi les could be generated in reasonable times. However, xenocrysts included in an exotic magma constitute a natural diffusion experiment of this type, in which concentration profi les of a suitable length can develop on geological as opposed to laboratory time scales (Costa and Chakraborty, 2004; Costa and Dungan, 2005). Here, we use a particularly propitious occurrence of diffusion profi les generated in olivine xenocrysts of upper-mantle composition immersed in a dioritic magma to constrain the rates of diffusion relative to Mg-Fe interdiffusion of several key trace elements (Li, Na, P, Ca, Sc, Ti, V, Mn, Co, Ni, and Y) at natural concentrations and at a temperature well below that practicable in the laboratory.

Ophiolites in China: their distribution, ages and tectonic settings

Abstract Ophiolites are widespread and abundant in China, where they lie along suture zones delineating major tectonic blocks. They range in age from Proterozoic to Tertiary and generally belong to the Palaeo-Asian, Qinling-Qilian-Kunlun, Tethyan and Circum-Pacific systems. A few possible Proterozoic ophiolites may reflect rifting of Rodinia at c. 800–1000 Ma. The Palaeo-Asian ophiolites, which range in age from Cambrian to Carboniferous, crop out abundantly in the northern parts of the country whereas the Qinling-Qilian-Kunlun ophiolites occur in north-central China, along the boundaries of the Tarim, North China and Yangtze blocks. Tethyan ophiolites are confined to southwestern China and Tibet whereas those of the Circum-Pacific belt are found in Taiwan and northeastern China. Chinese ophiolites are typically tectonically disrupted mélanges composed of isolated blocks of peridotite, gabbro and basalt. Sheeted dykes are rare or absent. Many of the ophiolites are compositionally complex, containing mixtures of island-arc tholeiite and boninite with lesser amounts of mid-ocean ridge basalt and ocean-island basalt. Most show evidence of having been formed or assembled in suprasubduction zone environments. However, Palaeo-Tethyan ophiolites generally lack suprasubduction zone signatures and may have formed in small, intracontinental basins. A rapidly expanding database of high-precision age dates and detailed geochemical analyses on Chinese ophiolites is providing new insight into the nature and timing of the tectonic events that shaped this part of Asia.

Was triassic continental subduction solely responsible for the generation of mesozoic mafic magmas and mantle source enrichment in the dabie-sulu orogen?

Mesozoic mafic rocks in the Dabie-Sulu orogen (DSO) are enriched in large-ion lithophile elements (LILE, e.g., Rb, K, Sr, Ba), relatively depleted in high-field strength elements (HFSE, e.g., Nb, Ta, P, Ti), and have strongly fractionated rare-earth elements (REE) and highly enriched Sr-Nd isotope ratios. These arc-type geochemical signatures have been generally ascribed to mantle source enrichment beneath the DSO by fluids/melts released from deeply subducted Triassic continental crust, i.e., part of the South China block (SCB). However, contemporaneous mafic rocks are widespread in the North China block (NCB) and possess similar elemental and Sr-Nd isotopic characteristics. Given that most of the NCB mafic rocks were emplaced at a considerable distance from the DSO, their arc-type geochemical features are unlikely to have been caused by Triassic continental subduction. Therefore, we argue that subduction of the SCB is a potential, but not necessarily the only, mechanism responsible for mantle enrichment beneath the DSO. We propose instead that the lithospheric mantle beneath the DSO and NCB was enriched prior to Triassic subduction, and that generation of the mafic rocks in both regions was due to a shared, regional-scale geological event, which we attribute to post-collisional delamination of the mantle lithosphere.

Formation of Late Archean High-δ18O Diorites through Partial Melting of Hydrated Metabasalts

&NA; Insight into the formation of Archean trondhjemite‐tonalite‐granodiorite (TTG) and sanukitoids is essential for understanding Archean crustal evolution and tectonic styles; however, their exact source and petrogenesis are still open to debate. Detailed chemical compositions of minerals, whole‐rocks and whole‐rock Sm‐Nd isotopes, zircon U‐Pb ages and Hf‐O isotopes of the Zhulagou (ZLG) diorites and mafic enclaves from the Yinshan Block of the North China Craton are used to investigate the petrogenesis of the diorites and related geological processes. The ZLG diorites, which formed at ˜2520 Ma, have moderate SiO2 (59·4–65·5 wt %) and Mg# (49–52), high Al2O3 (15·6–20·6 wt %), Cr (90·4–438 ppm), Ni (15·0–95·9 ppm), Sr (436–882 ppm) and Ba (237–1206 ppm) contents, and also fractionated rare earth element patterns (REE; LaN/YbN = 9·1–40·5) and depleted high field strength elements (HFSE) such as Nb, Ta and Ti, similar to Archean sanukitoids. The diorites may contain cumulus plagioclase and do not represent pristine melts, as indicated by their high Al2O3 contents and positive Eu and Sr anomalies. They have evolved whole‐rock Nd isotope compositions [&egr;Nd(t) = 1·0–2·3, TDM = 2·8–2·7 Ga], variable zircon &egr;Hf(t) (‐1·6 to +6·0) and high zircon &dgr;18O (˜9·0 ± 0·4‰, 2SD) values, indicating that the parental magma was most probably produced by partial melting of 2·8–2·7 Ga mafic crust. The high‐&dgr;18O character of the zircons was probably inherited from the magma source of hydrated metabasalts, which had elevated &dgr;18O owing to previous low‐temperature alteration, rather than resulting from granulite‐facies metamorphism or wall‐rock contamination. Coeval mafic enclaves (˜2525 Ma) in the diorites are dominantly composed of clinopyroxene, amphibole and orthopyroxene. They have low SiO2 (46·5–50·3 wt %) and high CaO (8·3–12·3 wt %), Cr (647–1946 ppm) and Ni (197–280 ppm) contents, convex light REE patterns, and similar Nd isotope compositions to the host diorites. In addition, clinopyroxene and amphibole in the mafic enclaves have major and trace element compositions similar to those of the diorites. These observations collectively indicate that the mafic enclaves probably represent cumulates from the diorite magma. The diorites and enclaves experienced regional granulite‐facies metamorphism at ˜2500 Ma, soon after their emplacement. The late Archean large‐scale crustal anatexis and high‐grade metamorphism within the Yinshan Block of the NCC probably occurred at the root of a continental arc.