Sedimentology, Stratigraphy and Detrital Zircon Geochronology of Mesoproterozoic Strata in Northern Helan Mountains, China (original) (raw)
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Geological Society of America Bulletin, 2023
The tectonics and landscape of SE China experienced significant changes throughout the Mesozoic and early Cenozoic, largely in response to variations in the slab dynam-ics of the paleo-Pacific plate, which was subducting beneath continental Asia. We investigated the Mesozoic Yong’an basin in western Fujian Province of SE China in comparison to the sedimentary records of coeval basins in the region to document how its clastic sediment types and their prov-enance varied through time during the Me-sozoic and what regional geologic processes may have controlled these variations. The average εNd value of samples from the Mid-dle Jurassic Zhangping Formation is −16.6, and its detrital zircons are dominated by 1800 Ma and 2000 Ma grains, sourced from the northern Wuyishan Mountains. These mountains underwent significant rock and surface uplift by the Middle Jurassic and became the main source of clastic sediments in SE China. The Lower Cretaceous Bantou Formation contains pyroclastic rocks and represents fluvial-lacustrine deposits with εNd values of −14.8 to −12.4 and abun-dant 160–120 Ma detrital zircons, sourced from Late Jurassic granitoid rocks, which were widely exposed at the surface in SE China by this time. The upper Lower and lower Upper Cretaceous Shaxian Forma-tion contains coarse-grained and poorly sorted sandstones-conglomerates with vol-canic and granitic rock fragments, and it rests unconformably on the Bantou For-mation. The Shaxian Formation represents fluvial- to alluvial-fan deposits, and its for-mation marks the timing of a rapid uplift of the paleo–Coastal Mountains. The Upper Cretaceous Chong’an Formation (>2000 m thick) contains abundant volcanic and gra- nitic rock clasts and represents alluvial-fan and fluvial deposits. The average εNd values of the Shaxian and Chong’an Formations range between −9.3 and −7.5, and their most abundant detrital zircon ages are be-tween 120 Ma and 80 Ma. By the end of the Late Cretaceous, the paleo–Coastal Moun-tains constituted a nearly 4-km-high mag-matic belt, with much of SE China situated in its rain shadow at a lower elevation to the north. Eocene–Oligocene sedimentary basin rocks in Taiwan have an average εNd value of −10.9 and abundant Phanerozoic detrital zircons. The sediment source for these rocks was the paleo–Coastal Mountains. The Mio-cene basinal strata in Taiwan have more negative εNd values (−13.0) and contain Jurassic–Cretaceous as well as abundant Paleoproterozoic and Neoproterozoic zir-cons, indicating that the Wuyishan Moun-tains were again the main sediment source later in the Cenozoic. Denudation rates in the SE margin of South China were high (0.12–0.10 km/yr) during the Cretaceous (140–60 Ma), while they were very low in SW China and in the interior of South China during the same period. These differ-ences confirm the existence of high coastal mountains in SE China until the Late Cre-taceous. Denudation rates in eastern South China, particularly the coastal areas, were very low (0.06–0.02 km/yr) during the late Cenozoic (30–0 Ma), whereas they were the fastest (0.14–0.16 km/yr) in the northern Nanling belt and the Yangtze block farther inland to the north, indicating the surface el-evation became higher in the western part of South China but lower in its eastern part in the late Cenozoic. This dynamic landscape evolution of SE China through multiple and major shifts throughout the Mesozoic and early Cenozoic was driven by the subduct-ing slab dynamics and the tectonics of the Tibetan Plateau.
In our study, the U-Pb age spectrum of detrital zircons from terrigenous sediments is used to determine the sources. The Middle and Upper Reaches of the Yangtze River drain the western Yangtze craton and eastern Tibetan plateau, and take lots of sediments to the Jianghan (江汉) plain. Detrital zircons collected from Zhoulao (周老) core provide an ideal sample for the provenance studies. Here we report LA-ICP-MS U-Pb ages of 125 detrital zircons from one sand sample in the core sediments, which is recovered from a depth of 86.76–88.58 m and the paleomagnetic age is about 800 kaBP, in Zhoulao Town. The 125 zircons reveal nine major age peaks of >3.0, 2.5–2.3, 2.0–1.7 Ga, 838–723, 453–415, 257, 212, 166 and <17 Ma. Only two zircons have ages of 14.8 and 16.5 Ma, their appearance indicates that the headwater of the Yangtze River had reached the Tibetan plateau about 800 ka ago.Our results also show that the Hanjiang (汉江) River did not influence the sediments of the Zhoulao core obviously at that time.
Chinese Science Bulletin, 2014
ABSTRACT This paper carries out studies about LA-ICP-MS U–Pb dating and Hf isotopic compositional analysis for detrital zircons in two metasedimentary samples that were taken from the Zhaochigou Formation-complex in the Helan Mountain. The results show that the Zhaochigou Formation-complex formed ~1.96 Ga, and it is a part of the Khondalite Series that is widespread in the North China Craton (NCC). The age spectrum of the detrital zircons indicates that the Zhaochigou Formation-complex shared the same provenance with the eastern edge of the Khondalite Series. Products of ancient magmatic activities within the Khondalite Belt may be the major source for the sedimentary materials that formed the Khondalite Series. Our age-dating results suggest that the Khondalite Series in the NCC may have experienced two stages of metamorphism, as the early stage occurred during 1.96–1.95 Ga, and the later stage occurred at about 1.87 Ga. The Hf isotopic data indicate that the Western Block of the NCC endured a stage of crustal growth at 2.5–2.3 Ga.
Gondwana Research, 2012
Whether any Grenvillian magmatic records are preserved in the North China Craton (NCC) is a key issue to understand the Proterozoic tectonic evolution of the NCC and its correlation to the supercontinent Rodinia. Meso- to Neo-proterozoic sedimentary series is well exposed in the NCC, but magmatic events in this period, especially of 1.3–1.0 Ga, have seldom been reported. New U–Pb isotopic dating and Hf isotopic composition analyses have been carried out in this study using SIMS and LA–ICP-MS methods on detrital zircons from sandstones of the Tumen Group in the Shandong Peninsula and quartz sandstones of the Sangwon System in the Phyongnam Basin, North Korea. The age populations of the detrital zircons of the Tumen Group are at ~2.5 Ga, ~1.85 Ga, ~1.7 Ga, ~1.58 Ga, ~1.5 Ga, ~1.36 Ga and ~1.2 Ga and those of the Sangwon System are at 1.88–1.86 Ga, ~1.78 Ga, 1.62–1.58 Ga, 1.46–1.41 Ga, ~1.32 Ga, ~1.17 Ga and ~980 Ma. Most of the age peaks of Neoarchean and Proterozoic correspond to the significant tectonic-magmatic-thermal events previously recognized in the NCC, revealing that the main provenances of the Tumen Group and the Sangwon System are Early Precambrian basement and Late Paleo- to Meso-proterozoic magmatic rocks of the NCC. Furthermore, the youngest detrital zircon ages of ~1.1 Ga from the Tumen Group and 984 Ma from the Sangwon System, as well as 910 Ma Rb–Sr whole rock isochron age of a limestone from the Tumen Group and 899 Ma mafic sills intruding the Sangwon System suggest that both groups were deposited in the Neoproterozoic, coevally with the Qingbaikou System in the Yanliao Aulacogen. The common zircon ages of 1.3–1.0 Ga from the Tumen Group and the Sangwon System, as well as the contemporaneous Penglai and Yushulazi Group in the eastern margin of the NCC, indicate that during the deposition of these sediments there have been significant contributions from Grenvillian magmatic rocks in the eastern NCC. This may provide clues to understand the possible relationship of the NCC and the supercontinent Rodinia. Moreover, the positive εHf (t) and ~2.8 Ga crust model ages of detrital magmatic zircons of 2.8–2.4 Ga suggest that there have been significant crustal growth at ~2.8 Ga in the eastern margin of the NCC, same as in other areas of the NCC.
Precambrian Research, 2009
The Paleo-Mesoproterozoic Xiong’er volcanic rocks along the southern margin of the North China Craton are lithologically and geochemically similar to those formed in subduction-related, continental margin volcanic arcs. The volcanic rocks are primarily composed of basaltic andesites and andesites, with minor dacites and dacitic rhyolites. Traditionally, the Xiong’er volcanic rocks have been divided from lower to upper into the Xushan, Jidanping and Majiahe Formations, but the ages of volcanic rocks in these formations have not been well constrained, which has hindered further understanding the tectonic significance of the Xiong’er volcanic belt at the southern margin of the North China Craton. SHRIMP and LA-ICP-MS U-Pb zircon analyses, combined with cathodeluminescence (CL) images, have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the Xiong’er volcanic rocks. SHRIMP and LA-ICP-MS U-Pb analyses on magmatic zircons from two basaltic andesite samples, one dacite sample and one rhyolite sample of the Xushan Formation, known as the lowest sequence of the Xiong’er volcanic rocks, indicate that the volcanic eruption of this Formation occurred at ∼1.78 Ga, but most xenocrystic/inherited zircons in these samples yielded 207Pb/206Pb ages ranging from 2.55 Ga to 1.91 Ga. Of three samples collected from the Jidanping Formation, two rhyolite samples (05XE015 and 05XE100) yielded weighted mean 207Pb/206Pb ages of 1778 ± 5.5 Ma and 1751 ± 14 Ma, respectively, similar to the ages of the volcanic rocks in the Xushan Formation, whereas one dacite sample (05XE066) gave a weighted mean 207Pb/206Pb age of 1450 ± 31 Ma, which is the youngest age obtained from the Xiong’er volcanic rocks. One andesite sample (06XS012) collected from the Majiahe Formation yielded two major age populations, with the older one at 1850 ± 5.9 Ma, interpreted as the age of the xenocrystic/inherited zircons, and the younger one at 1778 ± 6.1 Ma, interpreted as the age of the volcanic eruption to form the Majiahe andesite, coeval with the formation of most volcanic rocks from the Xushan and Jidanping Formations. These new SHRIMP and LA-ICP-MS U-Pb zircon data indicate that the traditional stratigraphic subdivision of the lower, middle and upper sequences of the Xiong’er volcanic rocks is not viable and that most of the Xiong’er volcanic rocks formed at 1.78–1.75 Ga, with minor felsic volcanic rocks erupting at ∼1.45 Ga. Similar-aged arc-related volcanic belts have also been found in the southern margin of North America, Greenland and Baltica, the western margin of the Amazonia Craton, the southern and eastern margins of the North Australia Craton, and the eastern margin of the Gawler Craton, which are considered to represent long-lived (1.8–1.3 Ga), subduction-related growth via accretion at key continental margins of the Paleo-Mesoproterzoic Columbia (Nuna) supercontinent.