Detrital zircon U-Pb-Hf isotopes and whole-rock geochemistry of neoproterozoic-cambrian successions in the Cathaysia Block of South China: Implications on paleogeographic reconstruction in supercontinent (original) (raw)
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Gondwana Research, 2011
This paper reports some new results from U-Pb geochronological, Hf isotopic and REE geochemical studies of detrital zircons in the Ordovician sandstones from South Jiangxi within Cathaysia. 426 groups of U-Pb age determinations define five major age populations: 2560-2380 Ma (a peak of 2460 Ma), 1930-1520 Ma (a peak of 1700 Ma), 1300-900 Ma (a major peak at 970 Ma and two subordinate peaks at 1250 Ma and 1130 Ma), 850-730 Ma (a prominent peak of 780 Ma) and 670-530 Ma (a major peak at 540 Ma and a subordinate peak at 650 Ma). We also report zircon U-Pb concordia age of 3.96 Ga, which is the oldest age so far obtained from Cathaysia. The age peak at 2460 Ma correlates with similar ages reported for Neoarchean global continental growth. The 1930-1520 Ma population broadly overlaps with the time of amalgamation and disruption of the Columbia supercontinent. The major age peak at 970 Ma and two secondary peaks at 1250 Ma and 1130 Ma reflect multiple tectonothermal events associated with the assembly of Rodinia. Similar ages are widely reported from the South China Craton (SCC). Our study reveals that the 850-730 Ma population is consistent with the breakup period of Rodinia, suggesting that the SCC within Rodinia began to break up since 850 Ma. Geologically, the evidence for this breakup event is widespread and presented by Neoproterozoic granites, bimodal igneous rocks, basic dyke swarms and formation of continental rift type basins. Our study also reveals a 670-530 Ma population that correlates well with the assembly of Gondwana during end Neoproterozoic. However, direct geological evidence for this event has not yet been found within the studied area. Furthermore, the Hf isotopic model age data suggest two major stages of crustal evolution within Cathaysia. The first is the event dated at 1.6-2.8 Ga and the second one at 3.5-3.9 Ga. The zircons show a large range of εHf(t) values from + 8.64 to − 30.54, suggesting that they have different origins with a similar age of crystallization. The fact that most detrital zircons show negative εHf(t) values suggests the ancient provenances of Cathaysia were dominated by reworked crustal materials.
The multiphase emplacements of granitoids and well-developed depositional sequences from Neoproterozoic to Mesozoic are exposed widely in the Wugongshan domain of South China provide important keys for the reconstruction of the pre-Mesozoic tectonic evolution of the Cathaysia Block. Here we present the results from geological investigations, detrital zircon U-Pb geochronology, in situ Hf-isotope and REE composition of the Ordovician meta-sandstones, together with the U-Pb age data on a Mesozoic pluton with an attempt to evaluate the petrogenesis and crustal evolution history. A group of 276 detrital zircon U-Pb ages are reported from four metasandstones which show five major age populations: 550 Ma). Among these, the most prominent age at 980 Ma fits well with the collisional event of Yangtze and Cathaysia Blocks. The second prominent age at 760 Ma correlates with a breakup of Rodinia, and the third one at 1680 Ma corresponds to the initial breakup of Columbia. For the fourth (peak of 2440 Ma) and fifth (peak of 550 Ma) ages, no corresponding magmatic rocks are exposed in the study area or in the adjacent regions, although the age populations broadly correspond to the global continental growth in the early Paleoproterozoic and the latest Neoproterozoic 'Pan-African' orogeny. We obtained 132 in situ Hf isotope compositions, among which 98 show negative εHf(t) values (− 36.88 to − 0.3) with two-stage model ages (T DM2 ) of 3.5-1.7 Ga (peak of 2.2 Ga), and 34 display positive εHf(t) values (+0.3 to +9.89) with crystallization ages of 1.5-1.1 Ga (peak of 1.4 Ga). Zircons from two Mesozoic granitic samples yield rather similar late Jurassic ages of 158.1 ± 2.1 Ma and 154.0 ± 3.9 Ma. All the corresponding εHf(t) values of zircons are negative (−24.2 to −4.2) and the Hf two-stage model ages range from 1.5 to 3.0 Ga with a peak of 1.68 Ga. Combined with previous age data on 120 Ma massive granitoids and 230 Ma on ductile sheared plutons, we consider that the magmatism in the Wugong complex started from the late Triassic, and reached a peak in the late Jurassic, but was gradually weakened since the early Cretaceous (130 Ma) and culminated at 120 Ma. Almost all the Wugongshan granitic plutons were derived from the partial melting of Mesoproterozoic crustal material, with little mantle components. We also discuss the provenance of the metasedimentary rocks and pre-Mesozoic crustal evolution of Cathaysia.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2016
The paleogeographic configuration of continental blocks around East Gondwana during the Neoproterozoicearly Cambrian is controversial. This study reports the U-Pb ages of detrital zircons and Nd isotopic composition of the Neoproterozoic-early Cambrian succession developed on the Precambrian basement in South Korea, which formed the southeastern portion of the Sino-Korean block (SKB) in its present configuration. Three stratigraphic units are addressed in this study: the Neoproterozoic Jangsan, early Cambrian Myeonsan, and earlymiddle Cambrian Myobong Formations. Both the Jangsan (white to pink quartz sandstone) and Myeonsan (dark gray ilmenite-rich sandstone/shale) formations are barren and are unconformably and conformably overlain, respectively, by the dark gray, fossiliferous fine-grained Myobong Formations. The Jangsan and Myeonsan Formations contain zircons with Archean-Paleoproterozoic ages, indicative of detritus derived from the local Precambrian basement. In contrast, the Myobong Formation is dominated by Mesoproterozoic to Neoproterozoic zircons, which are not represented in the local Precambrian basement. The Sm-Nd model ages of the Myobong Formation are younger than those of the underlying strata, indicative of significant changes in provenance during the deposition of this formation. Comparison with coeval sediment having Gondwana signatures in the southern margin of the SKB and the Tethyan Himalayan terrane strongly suggests that the Myobong Formation was derived from orogens in East Gondwana. The results of this study reveal that the timing of the Myobong Formation deposition marks the onset of a sedimentation episode on the southeastern margin of the SKB, which was related to the emergence of a vast source province in East Gondwana, possibly aided by the Cambrian transgression onto the SKB. In comparison with the published literature, we argue for the paleogeographic continuity of the SKB with the northern margin of East Gondwana, possibly between northwestern Australia and northeastern India during the Neoproterozoic-early Cambrian.
American Journal of Science, 2014
The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolitefacies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative Nd (t) values (؊9.7 to ؊13.7) and two-stage Nd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206 Pb/ 238 U ages of 435 ؎ 4 Ma, 424 ؎ 5 Ma, 428 ؎ 3 Ma and 427 ؎ 2 Ma. All the zircon Hf(t) values are negative (؊6 to ؊9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subductioncollision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.
Lithos, 2013
The leucogranites in the Baoshan Block of the Tethyan belt in Western Yunnan, are composed mainly of two-mica granite with subordinate muscovite granite. Here we present zircon U-Pb ages from four intrusions that show ages of 448-476 Ma suggesting that these rocks were emplaced during the Ordovician. The leucogranites are high-K calc-alkaline and strongly peraluminous, with K 2 O/Na 2 O N 1 and A/CNK = 1.12-1.54. These rocks are enriched in large-ion lithophile elements (LILEs) and light rare-earth elements (LREEs) [(La/Yb) N = 1.13-32.4] and Pb, and are depleted in high field-strength elements (HFSEs). They show similar chondritenormalized REE patterns, with negative Eu anomalies (Eu/Eu* = 0.03-0.46). A wide range of zircon ε Hf (t) values (−9.6 to −2.6) and varying Hf-isotope crustal model ages (2.1-1.6 Ga) are also observed. The geochemical signatures indicate that the leucogranites are S-type granites derived mainly from the anatexis of ancient crustal materials. The ages, geochemistry and tectonics in the Baoshan Block and the Lhasa Terrane are closely comparable, suggesting that the Baoshan Block might represent part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean. The Pinghe granites of the early phase in the Baoshan Block which are coeval with the Cambrian magmatism (ca. 492 Ma) identified in the central and southern Lhasa subterranes can be interpreted as products of the slab break-off associated with the subduction of proto-Tethyan oceanic lithosphere. However, the late leucogranite stocks are analogous to the North Himalayan leucogranites, which formed in a short-lived extensional setting caused by the slab break-off associated with the subduction-collision system.
Precambrian Research, 2014
Thick carbonate successions of Proterozoic age were deposited in the Huaibei and Huainan regions along the southern margin of the North China Block. These successions, including the Huaibei Group in the Huaibei region and the Huainan and Feishui groups in the Huainan region, have been correlated with the Qingbaikou Group, which outcrops in the Jixian area of North China and is the eponymous succession of the Qingbaikouan System (traditionally regarded as 1000-800 Ma). Recently, it has been shown that lower Qingbaikouan strata in the Jixian area may be as old as 1300-1400 Ma. By correlation, the Huaibei, Huainan, and Feishui groups may include Mesoproterozoic strata, but these units have also been interpreted as Tonian or Cryogenian-Ediacaran-Cambrian in age. These contradictory age interpretations hamper our ability to explore the rich paleontological and sedimentary records preserved in these rocks and to assess paleogeographic reconstructions. In this report, we describe new microfossils and stable isotope data (␦ 13 C carb , ␦ 18 O carb , ␦ 34 S CAS) from the Huaibei, Huainan, and Feishui groups. Possible vase-shaped microfossils from the Jiayuan Formation of the lower Huaibei Group and characteristic early Neoproterozoic acritarchs (particularly Trachyhystrichosphaera aimika) from the Gouhou and Liulaobei formations are consistent with a Tonian age for the Huaibei and Huainan groups. Similarly, carbon isotope trends in the Huaibei, Huainan, and Feishui groups are comparable with Tonian ␦ 13 C carb patterns in South Australia and Laurentia, with a plateau of moderately positive (1-5‰) values followed by pronounced negative ␦ 13 C carb excursion in the Gouhou Formation, which may be equivalent to the Bitter Springs anomaly in South Australia. ␦ 34 S CAS values of the Huaibei Group are mostly 25-40‰, similar to values from the stratigraphically equivalent Jingeryu Formation in the Jixian area. Overall, the combination of biostratigraphic, chemostratigraphic, and available radiometric data suggests that the Huaibei, Huainan, and Feishui groups are early Neoproterozoic (Tonian) in age.
Gondwana Research, 2008
Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ 13 C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ 13 C chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the δ 13 C carb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to −7‰ (negative excursion 1: N1). Subsequently, the δ 13 C carb increases continuously up to about + 5‰ at the upper part of the Nemakit-Daldynian stage. After this positive excursion, δ 13 C carb sharply decreases down to about −9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ 13 C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ 13 C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ 13 C profile best represents the global change in seawater chemistry. The minimum δ 13 C of the N1 (−7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13 C-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ 13 C rise up to +5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO 2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ 13 C (−9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.
Gondwana …, 2008
Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ 13 C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ 13 C chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the δ 13 C carb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to −7‰ (negative excursion 1: N1). Subsequently, the δ 13 C carb increases continuously up to about + 5‰ at the upper part of the Nemakit-Daldynian stage. After this positive excursion, δ 13 C carb sharply decreases down to about −9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ 13 C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ 13 C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ 13 C profile best represents the global change in seawater chemistry. The minimum δ 13 C of the N1 (−7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13 C-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ 13 C rise up to +5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO 2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ 13 C (−9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.