Bor-ming Jahn

A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

Abstract A-type granites are widely distributed in northeastern China (NE China). They were emplaced during three major episodes (the Permian, late Triassic to early Jurassic, and early Cretaceous) and evolved in different tectonic regimes. According to their mineralogical and geochemical characteristics, two subgroups of A-type granites (aluminous and peralkaline) can be recognized. The peralkaline subgroup contains alkali mafic minerals, such as riebeckite, arfvedsonite and sodic pyroxene, while the aluminous subgroup contains annite and Fe-rich calcic- or sodic-calcic amphibole. With respect to the aluminous subgroup, the peralkaline granites contain higher Rb, Ga and total rare earth elements (REE), but lower MgO, CaO, Al 2 O 3 , Ba and Sr. Based on the discrimination criteria of Eby [Geology 20 (1992) 641], the Permian and late Triassic to early Jurassic A-type granites belong to the A 2 (post-orogenic) type, whereas the early Cretaceous granites are of the A 1 (anorogenic) type. Nd isotopic compositions of these A-type granites indicate their derivation from a dominantly juvenile crustal source. Their origin is thought to have involved partial melting of an underplated lower crustal source. Because the generation of A-type granites requires high melting temperature, we propose models involving slab break-off, lithospheric delamination and extension. The Permian A-type granites have the same age range as those in eastern Junggar, southern Mongolia and central Inner Mongolia. They occur along a major suture and form a narrow belt between the north China and Siberian Cratons. We suggest that their formation was associated with post-collisional slab break-off. The late Triassic to early Jurassic A-type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late Paleozoic to early Triassic. The early Cretaceous A-type granites have an anorogenic affinity and were possibly associated with rifting in eastern China at this time, associated with the onset of paleo-Pacific subduction. Consequently, we conclude that the A-type granites in NE China were generated at three different times, involving multiple processes operative in different tectonic environments.

Crustal evolution of southeastern China: Nd and Sr isotopic evidence

We present a synthesis of crustal evolution in SE China based on extensive Nd and Sr isotopic data compiled from the literature for intrusive granitoids, volcanic, sedimentary and metamorphic rocks from three major tectonic units of SE China: Dabie, Yangtze and Cathaysia. Overall, igneous rocks of Phanerozoic ages possess initial eNd(T) values from −21 to +5 and depleted-mantle model ages (TDM) from 2.3 to 0.5 Ga. Sedimentary and metamorphic rocks have a TDM range of 3.3 to 1.1 Ga. The model age data indicate that the most important crustal formation took place in the Proterozoic, possibly with a very minor proportion produced in the late Archaean. This finding clearly suggests that the crustal evolution in SE China, including the Yangtze craton and Cathaysia, is distinctly different from the Archaean-dominated North China Block. The Dabie terrane, due to its lack of clear indication of Archaean crustal signal, might belong more logically to the Yangtze Block. The large database shows an oceanward younging of TDM for Phanerozoic igneous rocks in SE China, with the oldest ages found in Cathaysia Interior and the youngest in coastal Fujian and Taiwan. eNd(T) values show parallel systematic variations. Two NE-striking low-TDM zones characterized by high-REE granites in the middle of SE China are identified, but their tectonic significance is not yet clear. Within SE China, timing of the Yangtze-Cathaysia collision (Proterozoic vs. Mesozoic) has been controversial for many years. While the Mesozoic thrust model may be supported by a recent 40Ar/39Ar age study that reveals Triassic to Early Jurassic ages for mylonitic rocks from the same fracture zone, a Proterozoic collision model seems to be favoured because (1) abundant Neoproterozoic ages have been obtained recently for a variety of rocks occurring along the Jiangshan-Shaoxing fracture zone and for ophiolite suites of the Banxi Group, and (2) many lines of evidence have been found to argue against the Banxi Group as a melange complex and a long displaced thrust sheet. This issue is not yet resolved by geochronological means, but the present synthesis of Nd isotopic data and model ages and recent palaeomagnetic studies of Lower to Middle Triassic rocks appear to refute the hypothesis of Mesozoic collision.

Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic

The Central Asian Orogenic Belt (CAOB), also known as the Altaid Tectonic Collage, is characterised by a vast distribution of Paleozoic and Mesozoic granitic intrusions. The granitoids have a wide range of compositions and roughly show a temporal evolution from calcalkaline to alkaline to peralkaline series. The emplacement times for most granitic plutons fall between 500 Ma and 100 Ma, but only a small proportion of plutons have been precisely dated. The Nd-Sr isotopic compositions of these granitoids suggest their juvenile characteristics, hence implying a massive addition of new continental crust in the Phanerozoic. In this paper we document the available isotopic data to support this conclusion. Most Phanerozoic granitoids of Central Asia are characterised by low initial Sr isotopic ratios, positive e Nd (T) values and young Sm—Nd model ages (T DM ) of 300-1200 Ma. This is in strong contrast with the coeval granitoids emplaced in the European Caledonides and Hercynides. The isotope data indicate their ‘juvenile’ character and suggest their derivation from source rocks or magmas separated shortly before from the upper mantle. Granitoids with negative e Nd (T) values also exist, but they occur in the environs of Precambrian microcontinental blocks and their isotope compositions may reflect contamination by the older crust in the magma generation processes. The evolution of the CAOB is probably related to accretion of young arc complexes and old terranes (microcontinents). However, the emplacement of large volumes of post-tectonic granites requires another mechanism, probably through a series of processes including underplating of massive basaltic magma, intercalation of basaltic magma with lower crustal granulites, partial melting of the mixed lithologic assemblages leading to generation of granitic liquids, followed by extensive fractional crystallisation. The proportions of the juvenile or mantle component for most granitoids of Central Asia are estimated to vary from 70% to 100%.

Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China

Abstract The Emeishan flood basalt is a large igneous province erupted during the Permian–Triassic period in southwestern China. Based on petrographic, major and trace element, and Sr–Nd isotope data, the Emeishan basalts can be classified into two major magma types. These are: (1) a low-Ti (LT) type that exhibits low Ti/Y ( 500). The HT lavas can be further divided into three subtypes. HT1 lavas exhibit significantly high TiO2 (3.65–4.7%), Fe2O3* (12.7–16.4%), Nb/La (0.75–1.1), coupled with higher eNd(t) (1.1–4.8) and lower SiO2 (45–51%); HT2 lavas are compositionally similar to the HT1 lavas but show conspicuous depletion in U and Th. The HT3 type has higher Mg# (0.51–0.61) than the HT1 and HT2 lavas. It differs from the LT type in having higher TiO2 (∼3%) at comparable Mg#. Elemental and isotopic data suggest that the chemical variations of the LT and HT lavas cannot be explained by crystallization from a common parental magma. Instead, they may originate from different mantle sources under various melting conditions and underwent distinct differentiation and contamination processes. REE inversion calculations indicate that the HT magmas were generated by low degrees of partial melting (1.5%) of a mantle source that has eNd(t) of ∼+5 and 87Sr/86Sr(t) of ∼0.704 within the garnet stability field. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variations. In contrast, parental magmas of the LT type were generated by higher degree of partial melting (16%) of a distinct mantle source (eNd(t)≈+2, 87Sr/86Sr(t)≈0.705) around the spinel–garnet transition zone. The chemical evolution of the LT lavas is controlled by an olivine (ol)+clinopyroxene (cpx) fractionation. The Emeishan flood basalts may result from a starting mantle plume. The petrogenesis of both the LT and HT magmas was further complicated by contamination of upper crust and lithospheric mantle. While the HT1 lavas have experienced an AFC style of contamination in the upper crust, the HT2 lavas that mark with U–Th depletions may result from additional interaction with melts derived from a gabbroic layer near the crust–mantle boundary. In contrast, a temperature-controlled style of contamination was associated with the LT lavas. Our data show that both temporal and spatial geochemical variations exist in the Emeishan flood basalt province. The occurrence of thick LT lavas in the western part of the province may record the main episode of the flood basalt emplacement. In contrast, the less abundant overlying HT basalts may imply a waning activity of the plume. In fact, the HT basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT lavas may be a result of relatively thicker lithosphere and lower geotherm.

The Central Asian Orogenic Belt and growth of the continental crust in the Phanerozoic

Abstract Asia is the world’s largest composite continent, comprising numerous old cratonic blocks and young mobile belts. During the Phanerozoic it was enlarged by successive accretion of dispersed Gondwana-derived terranes. The opening and closing of palaeo-oceans would have inevitably produced a certain amount of fresh mantle-derived juvenile crust. The Central Asian Orogenic Belt (CAOB), otherwise known as the Altaid tectonic collage, is now celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic. It is composed of a variety of tectonic units, including Precambrian microcontinental blocks, ancient island arcs, ocean island, accretionary complexes, ophiolites and passive continental margins. Yet, the most outstanding feature is the vast expanse of granitic intrusions and their volcanic equivalents. Since granitoids are generated in lower-to-middle crustal conditions, they are used to probe the nature of their crustal sources, and to evaluate the relative contribution of juvenile v. recycled crust in the orogenic belts. Using the Nd-Sr isotope tracer technique, the majority of granitoids from the CAOB can be shown to contain high proportions (60 to 100%) of the mantle component in their generation. This implies an important crustal growth in continental scale during the period of 500–100 Ma. The evolution of the CAOB undoubtedly involved both lateral and vertical accretion of juvenile material. The lateral accretion implies stacking of arc complexes, accompanied by amalgamation of old microcontinental blocks. Parts of the accreted arc assemblages were later converted into granitoids via underplating of basaltic magmas. The emplacement of large volumes of post-accretionary alkaline and peralkaline granites was most likely achieved by vertical accretion through a series of processes, including underplating of basaltic magma, mixing of basaltic liquid with lower-crustal rocks, partial melting of the mixed lithologies leading to generation of granitic liquids, and followed by fractional crystallization. The recognition of vast juvenile terranes in the Canadian Cordillera, the western US, the Appalachians and the Central Asian Orogenic Belt has considerably changed our view on the growth rate of the continental crust in the Phanerozoic.

Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications

Abstract Chemical (major and trace element) and isotopic (Sr, Nd) analyses of the Luochuan loess-paleosol sequence in China were performed in order to examine the following problems: (1) potential source heterogeneity; (2) element behavior during pedogenesis; (3) paleoclimatic implications; and (4) the use of loess data to determine the average composition of the upper continental crust. New results clearly indicate that Rb, Sr, U and Ce abundances are severely depleted in paleosols relative to their “parental” loess. The behavior of these elements is strongly controlled by the breakdown of primary minerals such as carbonate (Sr) or by alteration processes in highly oxidizing environments (Ce and U). The pattern of element variations through the upper section of the Luochuan sequence mimics the magnetic susceptibility signal both in position and in intensity, thus reinforcing the pedogenesic origin of the enhanced magnetic susceptibility in paleosols. All loess samples display highly uniform REE patterns (except for Ce) characterized by the upper continental crust (UCC) ratios: ( La Yb ) N ≈ 10 and Eu Eu ★ ≈ 0.66. The paleosol REE patterns are similar to those of the loess, with the same LREE/HREE fractionation, but they also show distinct and variable negative Ce anomalies. Nd and Sr isotopic compositions are rather uniform in both loess and paleosols and do not vary with stratigraphic position (ϵNd = −10.5 to −9.2; 87 Sr 86 Sr = 0.715 to 0.719). The isotopic homogeneity in the Luochuan sequence strongly suggests a uniform source region during the entire period of deposition since ∼ 800 ka ago to the present. Several conclusions may be drawn from the present study: 1. (1) No geochemical distinction can be made between loess layers L1 to L7. The sources and the dust storm trajectories must have been essendally the same for the last 800 ka. 2. (2) Some elements, (e.g., Ca, Rb, Sr, U, Ce) are strongly fractionated by pedogenesis between loess and paleosols. Ce mobility in soils is clearly demonstrated but the process responsible for this leaching is still poorly understood. 3. (3) Systematic variations of element abundances and ratios between loess and paleosols can be used as chemical indicators for pedogenesic intensity and so for paleoclimatic change. These chemical indicators serve a function similar to that of oxygen isotopes in deep-sea sediments (pelagic foraminifera), or that of magnetic susceptibility in loess sequences. They are different recorders of paleoclimatic change. 4. (4) The striking uniformity of REE patterns and La Th ratios in the Luochuan loess and paleosols, as well as in loess worldwide, is an excellent starting point for estimating the average composition of the upper continental crust.

Plume-lithosphere interaction in generation of the Emeishan flood basalts at the Permian-Triassic boundary

The Emeishan flood volcanism that erupted at Permian-Triassic boundary time produced a large igneous province of at least 2.5 X 10 5 km 2 in the western margin of the Yangtze craton, southwestern China. The volcanic successions, suggested to have resulted from a starting mantle plume, comprise thick piles of basaltic flows and subordinate picrites and pyroclastics. The picrites, which have high magnesian contents (MgO ≊ 20–16 wt%), variable degrees of light rare earth element enrichment [(Ce/Yb) N ≊ 4–25] and heterogeneous isotope ratios [ϵ Nd ≊ (T) +4 to −4], are proposed to have been generated by mixing between the dominant plume-derived magmas and small amounts of lamproitic liquids from the continental lithospheric mantle.

Crustal evolution and Phanerozoic crustal growth in northern Xinjiang: Nd isotopic evidence. Part I. Isotopic characterization of basement rocks

The Central Asian Orogenic Belt (CAOB) is known as the most important site of juvenile crustal growth during the Phanerozoic. In order to examine the processes of such crustal generation and the role of Precambrian crust in the magma genesis, we conducted geochemical and Nd isotopic studies on both the basement rocks and Phanerozoic granites from the major tectonic terranes in northern Xinjiang: Altai, Junggar, Tianshan and North Tarim. In this paper only the results on the basement rocks are reported. The North Tarim Terrane is composed of Archean bimodal suite (TTG gneisses and amphibolites) and Proterozoic granitic gneisses. This terrane is a fragment of ancient continental crust and is tectonically dissociated from the CAOB. The other terranes in northern Xinjiang (Altai, Junggar, and Tianshan) belong to the CAOB. The Altai and Tianshan are composite terranes probably formed by accretion of Phanerozoic subduction complexes with entrained Proterozoic basement rocks as microcontinental blocks. Geochemical characteristics of amphibolites from Altai and Tianshan suggest their formation in island arc settings. Sm‐Nd model ages of the Tianshan basement rocks vary from 1.2 to 2.2 Ga, but mainly concentrated in 1.7‐ 2.1 Ga. Similarly, amphibolites and gneisses of the Altai terrane have TDM in two apparently discrete groups at 0.9‐1.5 Ga and 2.4‐2.6 Ga. The initial Nd isotope ratios or 1 Nd(T ) values indicate that a large proportion of basement gneisses from Tianshan were derived from remelting of Paleo- to Mesoproterozoic protoliths (1.7‐2.1 Ga). However, this is not the case for the Altai basement gneisses. Granitic gneisses and metasedimentary rocks (schists and phyllites) from the East and West Junggar terranes have much younger TDM ages of 0.7‐1.4 Ga. Presence of a minor Precambrian crustal component is possible as inferred from the model ages, but no data have shown Precambrian ages for the Junggar basement rocks. The model age and e Nd(T ) data support the idea that the Junggar terrane is composed of young island arc assemblages, and the Junggar Basin itself could be a trapped Paleozoic oceanic crust. Together with some reliable radiometric ages for basement rocks, the present isotopic data and TDM ages indicate that ancient microcontinental blocks constitute a significant proportion of the continental crust in northern Xinjiang, and probably also in the entire CAOB. q 2000 Elsevier Science B.V. All rights reserved.

Loess geochemistry and its implications for particle origin and composition of the upper continental crust

Abstract Chemical and Nd Sr isotopic compositions of loess samples from Argentina, Europe and Spitsbergen were analyzed to examine the nature of source terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical composition of the upper continental crust. From the relations between Na2O/Al2O3 and K/2O/Al2O3 ratios and CIA values (chemical index of alteration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE patterns are remarkably uniform with (La/Yb)N ≈ 10, which is characteristic of the upper continental crust (UCC). Negative Eu anomalies, expressed in Eu/Eu* ratios, vary from 0.65 for European loess to 0.8 for Pampean loess from Argentina. All loess deposits have nearly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractionated in size-fractions relative to the whole-rock values. Nd and Sr isotopic compositions clearly distinguish Argentinean loess (87Sr86Sr = 0.706–0.709, eNd(0) = −6to−1.5) from all other loess deposits (87Sr86Sr = 0.712–0.730, eNd(0) = −13to−8). The REE and isotopic results clearly indicate a significant contribution of young Andean volcanics to the Pampean loess deposits, whereas multi-recycled and well-mixed ancient sediments are principal sources for the other deposits. The present results reinforce the earlier conclusion reached by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim. Cosmochim. Acta 47 (1983) 1897–1905] that the average chemical composition of UCC can be obtained from eolian deposits as well as from fine-grained clastic sediments.

Geochemistry of the Xining, Xifeng and Jixian sections, Loess Plateau of China: eolian dust provenance and paleosol evolution during the last 140 ka

Abstract Loess and paleosol samples from three distant sections (Xining, Xifeng, Jixian) of the Chinese Loess Plateau were analyzed for chemical and Nd–Sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka. These three sections represent three different climatic conditions, from arid in the west (Xining) to more humid in the east (Jixian) across the Loess Plateau. Isotopic analyses show that all the three sections display a restricted range of 143 Nd/ 144 Nd ratios (or e Nd ≈−10±0.5), indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust. The loess and paleosol samples have typical upper crustal 87 Sr/ 86 Sr ratios (0.714–0.718), and their variation can partly be attributed to different carbonate/silicate ratios in the samples. REE data from the three sections are indistinguishable from each other; they show typical upper crust patterns characterized by (La/Yb) N ≈10 and Eu/Eu*≈0.65. The high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes. Various chemical indicators have been used to monitor the pedogenetic intensity in these sections. Although results for the Jixian section agree reasonably well with the known climatic conditions on the Loess Plateau, the other two sections yield results not in agreement with some well-accepted proxies like magnetic susceptibility or grain-size distribution. Some chemical proxy indicators established in our previous work on the Luochuan section, such as the systematic U-depletion and negative Ce anomalies in REE patterns in soils, are not clearly recorded in the present case. The discrepancy between different climatic proxies could be related to the effective soil water budget, temperature, as well as the rate of development of each proxy.