Sedimentary Response To The Late Caledonian-Early Hercynian Orogenesis, The North Qilian Orogen

Orogen and basin are the best basic structural types on the surface of Earth. The Orogenic Sedimentology is a subject that organically contact with them. With the modern geological theory, the Orogen Sedimentology employs the regional stratigraphy, sedimentology, tectonics, geochemistry, and geophysics to reveal the formation and distribution of basin, composition and evolvement, to reconstruct the paleogeography, the ancient ocean, the ancient structure and the lithosphere dynamics. Sedimentary response to orogenesis, also named to basin-range interaction, is an important content in the Orogenic Sedimentary and Sedimentary Geology. Provenance of synorogenic basin and its variation with the time are important ways and means by which to reconstruct the orogenesis and evolvement of basin.The North Qilian Orogenic Belt (NQOB) is located in the northeastern margin of the Tibetan Plateau. It succeeded the breakup of Rodinian Supercontinent, experiencing an evolution of rift in the early Cambrian and mature ocean in the late Cambrian-Ordovician, the active continent margin in the middle-late Ordovician, collision in Silurian-Devonian on the south margin of the North China Plate. The process of orogenesis and the attribute of synorogenic basin are still debated and the details of transfer of synorogenic basin is retain faintly. This paper studied the provenance of the terrigenous detrital rocks in Ordovician-Devonian by composition of fragment, sedimentary geochemistry and chronology of detrital zircon with the geological investigation in the field, which shed light on the several key scientific questions about tectonic evolution of orogen and synorogenic basin.Detrital zircons from the middle-late Ordovician were dated and show a wide age range from Archean to the early Paleozoic with prominent peak at around 2.5 Ga,1.6 Ga,1.0 Ga, and 0.47Ga. Combined the regional magmatic events, Archean and the early Proterozoic detritus mainly derived from the North China Craton (NC) to the north, whereas the Central Qilian Block (CQ) to the south likely provided the late Proterozoic grains. The early Paleozoic ages are in consistent with the time of magmatic arc related to subduction of oceanic crust. Precambrian grains were restricted to the upper of section, with Paleozoic grains descending from bottom to upper of section, which reflect the initial collision between the CQ and NC. The two youngest zircons from the bottom of section suggest that the time of initial collision is located in the 467Ma-450Ma.Composition of detrital rocks in Silurian shows quartz to ascend from Sunan to Shihuigou, to Shuiquan and from lower to upper in the section. Along the trending of orogen, rock fragments gradually descend from Sunan to Shihuigou, to Shuiquan, however, metamorphic rock fragments display a reverse variation. And metamorphic rock fragments in the Shuiquan area is evidently more than that in the Sunan and Shihuigou areas. In Shuiquan section, metamorphic rock fragments gradually descend from lower to upper, whereas those in the Sunan and Shihuigou sections ascend from lower to upper. The volcanic and sedimentary rocks fragments exist largely in the Sunan and Shuiquan areas and absent in the Shuiquan area. The volcanic rocks fragments gradually descend and the sedimentary rocks fragments ascend from lower to upper in Sunan and Shuiquan sections. In all studied sections, the content of feldspar in the eastern part of orogen is consistent with that in the western part of orogen.Geochemical analyses for detrital rocks in Silurian show:â‘ major elements in all samples usually lower than those of PAAS, indicating the low mature, except for mudstones in the Shuiquan area. Mudstones from Shuiquan area occupy the higher femic, k-feldspar and clay minerals contents, and lower SiO2 content, relative to those of sandstones. Samples from Sunan area show the descending SiO2 content and ascending femic composition from lower to upper of section, but the contents of K-feldspar and plagioclase are equivalent in the lower and upper of section. CaO2 contents of most samples linearly relative to CO2 contents of them, which suggest that CaO2 are contained mainly in the carbonate.â‘¡All samples from Sunan area and sandstones from Shuiquan area display the lower trace elements contents relative to PAAS in the Upper crust-normalized spider diagrams, whereas mudstones from Shuiquan area show the higher trace elements contents than those of PAAS. In the upper crust-normalized spider diagrams, all samples are depleted in Nb, Ta, and Sr elements. Samples from Sunan area and sandstones from Shuiquan are depleted in Rb, whereas mudstones from Shuiquan area are closed to the upper crust in Rb. The contents of Ni, Cr elements are enrichment with all samples from Sunan area and mudstones from Shuiquan area and are depleted with sandstones from Shuiquan area.â‘¢All of the samples display a right-inclined REE pattern after Chondrite-normalized. Rare earth elements (REE) of samples from the Hanxia Formation in Sunan area are similar to those of PAAS, with the lower contents of the Lujiaogou, Angzanggou, Qunnaogou Formation relative to PAAS. REE of samples from Shuiquan area show similar characters to maior elements and trace elements. REE of sandstones is lower than that of PAAS and REE of mudstones is higher than that of PAAS. All samples exhibit relatively strong negative Eu anomalies. These geochemical characteristics suggest the input of the felsic and mafic clast with minor granitic rocks into the eastern and wastern area. However, source of sediments in the eastern part underwent the weak chemical weathering and sedimentary recycle. Evidences combining tectonic discriminations and detrital zircon chronology suggest that sediments in the Sunan area were derived mainly from the metamorphic basement of CQ (1.6Ga,1.0Ga), the North Qilian Continental Arc (NQA) (0.45Ga), subducted complex (SC) (0.5-0.6Ga) and NC (2.5Ga), whereas sediments in Shuiquan area predominately came from the metamorphic basement of CQ and NC without the NQA. Combined the descending volcanic rocks fragments from lower to upper, alteration of provenance indicates that NQA was buried under the sediments from the two continents after the initial collision.The Middle-Lower Devonian Laojunshan Formation is a suite of molasse formed during the rapid uplift of the North Qilian Orogenic Belt (NQOB).41 sandstones have be sampled from the Sunan and Minle sections in the western sector and the Gulang and Jingyuan sections in the eastern sector of the NQOB belt. Geochemical analyses of those samples indicated:â‘ the MgO+Fe2O3T and Al2O3/SiO2 values are higher, and K20/Na2O ratios are lower in the western sector than those in the eastern sector.â‘¡All of them are depleted in Nb and Ta elements. The samples from the western sector are depleted in Rb element and enriched with Sc, Co, Ni, V, and Cr elements in the Upper Crust-normalized patterns. However, those from the eastern sector are depleted in Sr without enrichments of Sc, Co, Ni, V, and Cr.â‘¢All of the samples display a right-inclined REE pattern after Chondrite-normalized. But LaN/YbN and Eu/Eu* ratios of the samples from the western sector are lower than those of the samples from the eastern sector. These geochemical characteristics suggest the prominent input of mafic clast with minor granitic rocks into the Sunan area, felsic clast into the Gulang and Jingyuan area, both mafic and felsic clast into the Minle area. The angular shapes of gravels imply that these ill-sorted sediments were deposited near their sources without recycling. Geochemical features above also demonstrated that no major chemical weathering occurred for the western provenance, but deposits in the eastern sector resulted from low or middle degree chemical weathering. Evidences combining tectonic discriminations and detrital zircon chronology revealed that sediments in the Sunan area were derived mainly from NQA (0.5-0.4Ga) and SC (0.5-0.6Ga), while sediments in the Minle, Gulang, and Jingyuan areas were derived not only from NQA (0.5-0.4Ga) and SC (0.5-0.6Ga) but also from the basement of the metamorphic basement of CQ (1.6Ga,1.OGa) and NC (2.5Ga).The Upper Devonian Shaliushui Formation had recorded the end of the orogeny. Samples from the studied section located in the Shaliushui, Jingyuan County, contain major quartz and minor fragments and feldspar. All samples are depleted in Nb, Ta, and Sr elements and negative Eu anomalies. Geochemical data show that source underwent the middle chemical weathering and sediments did not undergo sedimentary recycle. Sediments were derived mainly from the felsic and mafic rocks, with minor granitic rocks. The tectonic discriminations and detrital zircon chronology revealed that sediments in the Upper Devonian were derived from NQA (0.5-0.4Ga) exhumed, and the metamorphic basement of CQ (1.6Ga,1.0Ga), SC (0.5-0.6Ga) and NC(2.5Ga) engulfed into orogen.Provenance and its alteration with the time can draw the following conclusion:1. The Late Ordovician detrital zircon chronology suggest that sediments in the eastern part of NQOB maily derived from the North Qilian Arc, no from the continental block at the two side of basin. However, until the late of Late Ordovician, a large of sediments from the two continental block, the Central Qilian Block to the south and the North China Plate to the north, were filled in the basin and buried the North Qilian Arc, which resulted into the decreasing sediments from the North Qilian Arc and the increasing sedioments from the two continental block with the time going. Integrated the outcrop of the Lujiaogou Formation, the Tianzhu Formation should be as sedimentary record of the initial collision.2. The composition, geochemistry and chronology of detrital rocks in Silurian show that provenance of eastern basin maily came from the Central Qilian Block and North China Plate and sediments underwent the chemical weathering and sedimentary recycle, whereas sediments without the chemical weathering and sedimentary recycle in the western basin mainly derived from the North Qilian Arc, the Central Qilian Block and North China Plate.3. The composition, geochemistry and chronology of detrital rocks in Devonian show that sediments from the Laojunshan Fromation in the western basin mainly derived from the North Qilian Arc without the chemical weathering, with no sediments from the continental block, and sediments did not undergo the sedimentary recycle. However, sediments deposited in the eastern part of basin mainly came from the North Qilian Arc, the Central Qilian Block and North China Plate, with the chemical weathering and sedimentary recycle. Provenance of the Upper Dovonian Shaliushui Formation inherited that of the Laojunshan Formation in the eastern basin. Sediments mainly were supplied by the North Qilian Arc, the Central Qilian Block and North China Plate engulfed into the orogenic belt.4. The detrital zircons with U-Pb ages of 8-10 Ga occur in the Ordovician-Devonian sediments, which indicated that the source of them, the Central Qilian block, similar to the Yangtze Plate and dissimilar to the North China Plate on the basis of sedimentology.5. The initial collision between the Central Qilian block and North China Plate happened in the Middle-Late Ordovician (467-450 Ma) at the eastern part of orogen, Wuweu area. This event resulted in the collisional conglomerate (the Tianzhu and Gulang Formation) and the unconformity between the middle and late Ordovician, named the Gulang Movement. The sequent collision occurred at the Sunan area in the Early Silurian and formed the Lujiaogou Formation conglomerate. Thus collision between the Central Qilian Block and North China Plate show that the orogeny of NQOB was diachronous in the trending direction from east to west.6. The provenance of the lower and middle Devonian Laojunshan Formation and the distributions of Silurian-Devonian jointly suggest that the orogeny of NQOB was diachronous in the trending direction and the eastern sector had stronger tectonic intensity compared to the western sector, which resulted into the Late Devonian collapse in the eastern part of orogen7. The synorogenic basin in the North Qilian-Hexi Corridor belt had transferred from the Early-middle Ordovician retroarc basin to the Late Ordovician remnant retroarc basin, to Silurian-Devonian foreland basin and the remnant retroarc basin and foreland basin indeed coexisted in the Late Ordovician-Early Silurian.