Caixia Feng

Cenozoic high Sr/Y volcanic rocks in the Qiangtang terrane, northern Tibet: geochemical and isotopic evidence for the origin of delaminated lower continental melts

Geochemical and Sr–Nd–Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO 2 contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29–58) and Sr/Y (43–92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low e Nd (t) values (−3.8 to −5.0) and highly radiogenic Sr (( 87 Sr/ 86 Sr) i = 0.706–0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers.

Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province, eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (87Sr/86Sr) i ranging from 0.7082 to 0.7087, low ϵNd(t) values from −17.0 to −17.5, 206Pb/204Pb from 17.14 to 17.18, 207Pb/204Pb from 15.44 to 15.55, and 208Pb/204Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ∼120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.

Zircon U–Pb age and Sr–Nd–Hf isotopic constraints on the age and origin of Triassic mafic dikes, Dalian area, Northeast China

Post-orogenic mafic rocks from Northeast China consist of swarms of dolerite dikes. We report a new U–Pb zircon age, as well as whole-rock geochemical and Sr–Nd–Hf isotopic data. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon analysis yielded an age of 210.3 ± 1.5 million years (i.e. Triassic) for these mafic dikes. Most Dalian mafic rocks exhibit low K2O + Na2O contents, and span the border between alkaline and calc-alkaline rock associations in the total alkali–silica diagram. The investigated dikes are also characterized by relatively high (87Sr/86Sr)i ratios (0.7061–0.7067) and negative ϵNd (t) (−4.7 to −4.3) and ϵHf (t) values (−4.1 to −1.1), implying that they were derived from an enriched lithospheric mantle source. The mafic dikes are characterized by relatively low MgO (4.65–5.44 wt.%), Mg# (41–44), and compatible element content [such as Cr (89.9–125 ppm) and Ni (56.7–72.2 ppm)], which are the features of an evolved mafic magma. No evidence supports the idea that the mafic rocks were affected by significant assimilation or crustal contamination during emplacement. We conclude that the dolerites formed in a post-orogenic extensional setting, related to lithospheric delamination or ‘collapse’ of the Central Asian Orogenic Belt (CAOB), also termed the Xingmeng Orogenic Belt in China.

K-Ar Ages and Geochemical + Sr-Nd Isotopic Compositions of Adakitic Volcanic Rocks, Western Shandong Province, Eastern China: Foundering of the Lower Continental Crust

K-Ar age, geochemical, and Sr-Nd isotopic data are reported for western Shandong volcanic rocks with the objectives of deciphering the origin and dynamic significance of Mesozoic magmatic events within the North China craton. K-Ar ages indicate eruption of the volcanic rocks between 116.2 ± 1.1 and 120.3 ± 1.1 Ma. The studied rocks include andesite and dacite that have SiO2 contents ranging from 59.3 to 65.9 wt% and Al2O3 contents from 15.3 to 16.8 wt%; K2O and Na2O contents range from 3.4 to 5.1 wt% and 3.1 to 3.8 wt%, respectively, indicating that they belong to the K-rich series. These rocks are depleted in heavy rare-earth elements (HREE) and variably enriched in light rare-earth elements (LREE). They have positive Ba, Sr, and K and negative Nb, Ta, and Ti anomalies. The rocks have high Sr ranging from 492 to 1665 ppm and relatively low Yb (0.77-1.75 ppm) and Y contents (9.5-17.1 ppm), resulting in high Sr/Y (52-102) and La/Yb ratios (28-45), characteristic of adakites. εNd(t) and initial Sr isotopic compositions range from -13.2 to -10.3 and from 0.7089 to 0.7098, respectively. These geochemical features are consistent with an origin from adakitic magmas that were likely derived from dehydration melting of a delaminated lower crust. The relatively high MgO, Cr, and Ni contents in the analyzed rocks suggest that interaction between melts and peridotite may have occurred before eruption. This study provides further evidence for foundering of lower crust accompanied by lithosphere thinning beneath the North China craton (NCC).

Geochemical and Sr–Nd–Pb isotopic compositions of Mesozoic mafic dikes from the Gan-Hang tectonic belt, South China: petrogenesis and geodynamic significance

Mesozoic mafic dikes in the Gan-Hang tectonic belt (GHTB) provide an opportunity to explore both the nature of their mantle source(s) and the secular evolution of the underlying Mesozoic lithospheric mantle in the region. The geochronology and primary geochemical and Sr–Nd–Pb isotopic compositions of Group 1 (middle section of GHTB) and Group 2 (the rest of the section) dolerite dikes spanning the GHTB were investigated. K–Ar ages indicate that dikes of both groups were emplaced during the Cretaceous (131–69 Ma). The dikes are doleritic in composition and are enriched in both large ion lithophile elements (LILEs; e.g. Rb, Ba, and Pb) and light rare earth elements (LREEs), with a wide range of Eu anomalies, but are depleted in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti) and heavy rare earth elements (HREEs). Dikes sampled in the middle section of the GHTB (Group 1) show more pronounced REE differentiation and a greater contribution from crustal material than those from the east and west sections (Group 2) and are similar to GHTB volcanic rocks in exhibiting a slight enrichment in LREEs. The dolerites are further characterized by a wide range in 87Sr/86Sr i  = 0.7041–0.7110, 143Nd/144Nd i  = 0.511951–0.512758, ϵNd t  = –10.4 to +5.6, and Pb isotopic ratios (206Pb/204Pb i  = 18.1–18.3, 207Pb/204Pb i ≈ 15.6, and 208Pb/204Pb i  = 38.2–38.7). The dikes have undergone fractional crystallization of olivine, clinopyroxene, plagioclase, and Ti-bearing phases, except for dikes from the Anding area, which possibly experienced fractionation of plagioclase. Geochemically, all the dike samples originated from mantle sources ranging in composition from depleted to enriched that contained a component of foundered lower crust; crustal contamination during the ascent of these magmas was negligible. In the context of the late Mesozoic lithospheric extension across South China, mafic dike magmatism was likely triggered by the reactivation of deep faults, which promoted foundering of the lower crust and subsequent mantle upwelling in the GHTB.

U–Pb zircon geochronology, geochemical, and Sr–Nd isotopic constraints on the age and origin of basaltic porphyries from western Liaoning Province, China

Basaltic porphyries from the northeast North China craton (NCC) provide an excellent opportunity to examine the nature of their mantle source and the secular evolution of the underlying mantle lithosphere. In addition, the study helps to constrain the age and the mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic analyses of a suite of mafic lavas. Detailed laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yielded an age of 223.3 ± 1.1 million years, which we regard as representing the crystallization age of the basaltic porphyries. The bulk-rock analysed samples are enriched in both large ion lithophile elements (LILEs) (i.e. Ba, Sr, and Pb) and light rare earth elements (LREEs), but depleted in high field strong elements (HFSEs) (i.e. Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements (HREEs), without significant Eu anomalies (Eu/Eu* =  089–0.98). The basaltic porphyries have undergone low degrees (∼5%) of partial melting of a garnet-bearing lherzolite mantle. The rocks display very uniform (87Sr/86Sr) i (0.70557–0.70583) and negative ϵNd (t) values (–11.9 to –10.1). These features indicate that the western Liaoning basaltic porphyries were derived from a common enriched lithosphere mantle that had previously been metasomatized by fluids related to subduction of Palaeo-Asian sedimentary units. However, the mafic melts were not affected to a significant degree by crustal contamination. Based on earlier studies, these findings provide new evidence that the northeast margin of the NCC had undergone a phase of post-orogenic extensional tectonics during the Middle Triassic. Furthermore, lithospheric thinning occurring across the northern NCC might have been initiated during Early Triassic times and was likely controlled by the final closure of the Palaeo-Asian Ocean, as well as the collision of Mongolian arc terrenes with the NCC.

U–Pb zircon ages, geochemical and Sr–Nd–Pb isotopic constraints on the dating and origin of intrusive complexes in the Sulu orogen, eastern China

TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean 206Pb/238U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (87Sr/86Sr)i ranging from 0.7083 to 0.7093, low ϵNd(t) values from ϵ14.6 to − 19.2, 206Pb/204Pb = 16.65–17.18, 207Pb/204Pb = 15.33–15.54, and 208Pb/204Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.

Experimental Synthesis of the Stibnite-Antimonselite Solid Solution Series

Experiments on the Sb-S-Se system were conducted at 300°C, and a continuous stibnite-antimonselite binary solid solution was established. By substituting S for Se, the compositions of S-rich and Se-rich endmembers were confirmed as Sb2S3 and Sb2Se3, respectively. Based on Se/(S+Se) ratios of microprobe analyses, binary stibnite-antimonselite solid solutions are defined as stibnite, selenium stibnite, sulfur antimonselite, and antimonselite. Microhardness of the stibnite subseries (Sb = 60.11-72.58, S = 13.20-27.63, and Se = 0.00-27.23 wt%) and the antimonselite subseries (Sb = 49.29-59.25, Se = 28.89-51.94, S = 0.00-12.10 wt%) varies from 112.95 to 127.72 kg/mm2. The variation of Se concentration is continuous throughout the series, confirming a random substitution of Se for S. Crystallographic parameters obtained from the series vary as follows: a = 1.123375-1.163890 nm, b = 1.132502-1.179553 nm, c = 0.383914-0.398071 nm, D = 4.593-5.896 g.cm-3, and V = 0.488425-0.546500 nm3. As evident from the above data, the higher the Se concentration, the larger the crystallographic parameters. The Sb-S-Se binary solid solutions obey Vegards law.

Geochemical constraints on the origin and environment of Lower Cambrian, selenium-rich siliceous sedimentary rocks in the Ziyang area, Daba region, central China

The Ziyang area is one of the two major regions of central China subjected to selenium (Se) poisoning. Systematic studies of Se contents of different lithologies from this area indicate that Lower Cambrian, carbonaceous, and siliceous strata host the highest Se contents (with Se contents of up to 278 ppm). We have investigated their geochemical characteristics (major and trace elements, and Si and O isotopes), origin, and sedimentary environment of formation. The siliceous rocks are characterized by a wide range in major elements, and are enriched in Se, Ba, Cu, Ni, V, As, Sb, and U relative to average continental crust. They are also enriched in light rare earth elements relative to heavy rare earth elements (LaN/YbN = 1.64–35.7) and show weak or moderate negative Ce anomalies and strong positive Eu anomalies. δ30SiNBS-28 and δ18OV-SMOW values range from –0.3‰ to 0.6‰ and 16.1‰ to 21.7‰, respectively. The homogenization temperatures of inclusions within the studied samples range from 113°C to 319°C, and their salinities from 1.2 to 13.7 wt.% NaCl equivalent. Our results suggest that the studied siliceous rocks resulted from hydrothermal sedimentation in a relatively anoxic semi-deep sea sedimentary environment. The hydrothermal fluid responsible for Se-mineralization involved the mixing of low-temperature high-salinity fluid, low-temperature low-salinity fluid, and a high-temperature low-salinity basinal fluid in the NaCl–(KCl)–H2O system.

Geochemical, Sr–Nd–Pb isotope, and zircon U–Pb geochronological constraints on the origin of Early Permian mafic dikes, northern North China Craton

Dolerite dike swarms are widespread across the North China Craton (NCC) of Hebei Province (China) and Inner Mongolia. Here, we report new geochemical, Sr–Nd–Pb isotope, and U–Pb zircon ages for representative samples of these dikes. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb analysis yielded consistent Permian ages of 274.8 ± 2.9 and 275.0 ± 4.5 Ma for zircons extracted from two dikes. The dolerites have highly variable compositions (SiO2 = 46.99–56.18 wt.%, TiO2 = 1.27–2.39 wt.%, Al2O3 = 14.42–16.20 wt.%, MgO = 5.18–7.75 wt.%, Fe2O3 = 8.03–13.52 wt.%, CaO = 5.18–9.75 wt.%, Na2O = 2.46–3.79 wt.%, K2O = 0.26–2.35 wt.%, and P2O5 = 0.18–0.37 wt.%) and are light rare earth element (LREE) and large ion lithophile element (LILE, e.g. Rb, Ba, and K, and Pb in sample SXG1-9) enriched, and Th and high field strength element (HFSE, e.g. Nb and Ta in sample SXG1-9, and Ti) depleted. The mafic dikes have relatively uniform (87Sr/86Sr)i values from 0.7031 to 0.7048, (206Pb/204Pb)i from 17.77 to 17.976, (207Pb/204Pb)i from 15.50 to 15.52, (208Pb/204Pb)i from 37.95 to 38.03, and positive ϵNd(t) (3.6–7.3), and variable neodymium model ages (TDM1 = 0.75–0.99 Ga, TDM2 = 0.34–0.74 Ga). These data suggest that the dike magmas were derived from partial melting of a depleted region of the asthenospheric mantle, and that they fractionated olivine, pyroxene, plagioclase, K-feldspar, and Ti-bearing phases without undergoing significant crustal contamination. These mafic dikes within the NCC formed during a period of crustal thinning in response to extension after Permian collision between the NCC and the Siberian Block.