Department of Applied Geology Timing and Kinematics of Mesozoic-Cenozoic Mountain Building and Lithospheric Thinning in the Eastern North China: Constraints from Geochronology and Thermochronology Liping Liu This thesis is presented for the Degree of Doctor of Philosophy Of Curtin University October 2014 Declaration DECLARATION To the best of my knowledge and belief this thesis contains no material previously published by any other person except where due acknowledge has been made. This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. Liping Liu Date: 17/10/2014 I Abstract ABSTRACT The Dabie-Sulu orogenic belt in central eastern China is best known for the widespread occurrence of ultrahigh-pressure (UHP) metamorphic rocks, a protolith of which was subducted to >100 km depths beneath the North China block, overprinted by UHP metamorphism, and finally exhumed to the surface. The Sulu UHP belt is offset from the Dabie UHP metamorphic belt by approximately 500 km of left-lateral strike-slip displacement along the Tan-Lu fault. It remains controversial as to what role the Tan-Lu fault played during collision-exhumation along the Dabie-Sulu orogenic belt. Current models in the literature include the transform fault model, lithospheric indentation model, crustal detachment model and rotational collision model. The thermal history of the Sulu UHP belt, and surrounding regions such as the Jiaobei region in the north and the Luxi region in the west, may thus provide valuable insights into the collision process. In addition, eastern North China is also one of the best studied regions for lithospheric thinning, which is commonly believed to have occurred since the Mesozoic. This event should also have been recorded in the regional thermal history. This study utilizes zircon U- Pb geochronology and multiple thermochronometry methods including mica and hornblende 40Ar/39Ar, zircon and apatite fission-track, and zircon and apatite (U- Th)/He dating to more fully constrain the thermal evolution of the region, thus shedding new light on the collision process between the North China and the South China blocks, as well as on the lithospheric thinning process. 40Ar/39Ar and zircon (U-Th)/He data show that the Sulu UHP terrane experienced a prominent cooling event at ca. 210–160 Ma. This event is interpreted as representing an erosional response to northward thrust-driven uplift of the UHP rocks and can be best explained by the crustal detachment model. A subsequent episode of exhumation took place between ca. 125 Ma and 90 Ma as recorded by zircon (U-Th)/He data. This event was more pronounced in the northern section of the UHP terrane, whereas in the southern section, the zircon (U-Th)/He system retained Jurassic cooling ages of ca. 180–160 Ma. The mid-Cretaceous episode of exhumation is interpreted to have resulted from the removal of a thickened enriched mantle lithosphere, and crustal extension. A younger episode of exhumation was recorded by apatite fission-track and (U-Th)/He ages at ca. 65–40 Ma. Both the 125– II Abstract 90 Ma event and the 65–40 Ma event are interpreted to represent episodic thinning of the lithosphere along the Sulu orogenic belt in an extensional environment, likely linked to the roll-back of the Western Pacific subduction system Thermochronologic and geochronologic analyses performed in the Jiaobei region, provide additional constraints on the timing of deformation and exhumation pertaining to the Mesozoic collision between the South and North China blocks. Three distinct episodes of deformation (D1, D2 and D3) were previously found in the Jiaobei region. D1 features penetrative foliations and mineral stretching lineations in Precambrian metamorphic basement and is constrained to have taken place at ca. 40 39 1974–1834 Ma as recorded by muscovite and hornblende Ar/ Ar data. D2 is marked by cleavages transposing the primary bedding in the Neoproterozoic-upper Paleozoic Penglai Group, and by WNW trending, NE-verging folds in the Precambrian basement. Exhumation at ~260 Ma could be related to D2 deformation and represent the tectonic exhumation of the overriding plate resulting from initial continental collision. D3 deformation is characterized by NNE-trending inclined folds subparallel to the dominant strike of foliations in the Sulu orogenic belt. Zircon (U-Th)/He data indicate westward-advancing exhumation from 196 ± 9 Ma to 164 ± 7 Ma, which is partly concomitant with exhumation of the ultrahigh-pressure rocks in the Sulu orogenic belt. The latter two directions of structural orientation and associated exhumation can be best explained by a crustal detachment model. In addition, mica 40Ar/39Ar, zircon fission-track and (U-Th)/He data from Upper Jurassic-Lower Cretaceous granitoids reveal exhumation episodes in an extensional context at ~130–90 Ma and at ~65–40 Ma, again testifying to episodic lithospheric thinning. Fission-track and (U-Th)/He results on zircon and apatite reveal multiple tectonic events in the Luxi region during the Phanerozoic. Zircon fission-track ages of 442–309 Ma and the large dispersion of zircon (U-Th)/He ages (738–484 Ma), along with the absence of the Upper Ordovician–Lower Carboniferous strata, suggest that this region underwent slow denudation from the Late Ordovician to early Carboniferous. Triassic to Late Jurassic crustal shortening in the region did not fully exhume the Archean rocks to depths of ca. 8 km, indicating that crustal exhumation during the SCB-NCB collision was not as severe here as in the region east of the Tan-Lu fault. During the Early Cretaceous, extension-related denudation exhumed the Archean rocks above ca. 8 km and yielded uniform single-grain ZHe ages. III Abstract Apatite fission-track and (U-Th)/He ages of 60–40 Ma reveal an episode of rapid exhumation during early Cenozoic. The latter two episodes of exhumation, synchronous throughout the entire study region, reflect episodic lithospheric thinning in eastern North China. Widespread late Mesozoic granitoids in the Jiaobei region are potential records of crustal thickening associated with the North China-South China collision and/or subsequent lithospheric thinning in the North China block. To unravel the petrogenesis of two major episodes of granitoid formation, in-situ zircon U-Pb-Hf isotopic and whole-rock geochemical analyses were carried out on the Linglong- Luanjiahe granites and the Guojialing granodiorites, and on mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U-Pb dating revealed that the Linglong granites and mafic enclaves crystallized in the Late Jurassic (157–148 Ma). They show high concentrations of Ba (1505–2809 ppm) and Sr (530–1544 ppm), low Y contents (< 20 ppm), high LREE contents with variable LREE/HREE ratios, and negative HFSE anomalies. Strongly negative zircon εHf(t) values (−27 to −18) indicate that the Linglong granites were sourced from an Archean lower continental crust. The Linglong granites and enclosed MMEs are cogenetic. Fractionation of hornblende and allanite mainly controlled the REE pattern. All these geochemical and isotopic features, in combination with low magma temperatures (645–780 °C), suggest that the Linglong granites were unlikely to have formed by dehydration melting of amphibolite with garnet as a residue. High concentrations of Ba and Sr indicate high solubility of plagioclase in water-rich magmas. Therefore, the Linglong granites are interpreted to have been formed by water-fluxed melting of biotite gneiss or the lower continental crust. The Luanjiahe granites, with a zircon U-Pb age of 159 ± 1 Ma, may have been derived from melting of younger sources as revealed by their higher εHf values (−18 to −11). The crustal detachment model can best accommodate the diverse conditions required for generation of the granitoids, such as multiple sources, temperature build up, and external water responsible for the geochemical patterns of the Linglong-Luanjiahe granites. The Guojialing granodiorites were emplaced during 127–124 Ma with contemporaneous mafic igneous rocks. The granodiorites and MMEs also have high LREE/HREE ratios and negative HFSE anomalies. The occurrence of MMEs and less negative εHf(t) values (−23 to −10), relative to the Linglong granites, suggest the involvement of mantle components in the sources. Fractionation of hornblende and titanite probably played IV Abstract a major role in the depletion of REE contents at high silica contents. Similar εNd(t) values, higher initial 87Sr/86Sr ratios and hornblende fractionation trends, relative to contemporaneous mafic rocks, indicate that the Guojialing granodiorites were differentiated from an enriched mantle-derived mafic magma with minor crustal contamination. This magmatic event was likely linked to the thinning of an enriched mantle lithosphere in the Early Cretaceous. Overall, the pre-160 Ma exhumation history in the Sulu orogenic belt and the Jiaobei region, together with activities in the Tan-Lu fault at 221–181 Ma and at ~160 Ma, can best be explained by the crustal detachment model. The two episodes of exhumation, in the Early Cretaceous and the early Cenozoic, likely reflect two episodes of lithospheric thinning in the eastern North China block. The lithospheric thinning may have been controlled primarily by extension due to roll-back of the subducting Western Pacific oceanic slab. V Acknowledgements ACKNOWLEDGEMENTS This thesis would not have been accomplished without the support and contributions from the following individuals over the past four years. First and foremost, I would like to thank my principal supervisor, Prof. Zheng-Xiang Li, for providing me the opportunity to work on this interesting project. I appreciate the adequate freedom that he offered regarding my research topic. His constructive criticism and inspiring discussions opened my mind and helped me to learn a lot in terms of fieldtrip planning, data presentation and ways of thinking. I am always amazed at the way he thinks about a certain scientific question.