The Late Mesozoic Magmatism Along The South Segment Of The Tan-Lu Fault Zone And Its Implication For Lithospheric Thinning

In this paper, Chronological, geochemical and Sr-Nd-Pb isotopic analyses have been carried out on the Late Mesozoic igneous rocks including Chuzhou volcanic rocks, Chaohu intrusive rocks and Lujiang volcanic rocks along the southern segment of the Tan-Lu fault zone. Based on these results, this paper demarcated formation time of these igneous rocks, summarized nature of their petrology and geochemistry, discussed deep processes of the magmatism, compositions of magma sources, and magmatic evolution features. Based on regional comparsions, this paper also analyzed patterns and characteristics of the lithosphere thinning beneath the Tan-Lu fault zone, revealed the importance and function of such a large-scale fault zone during the lithosphere thinning of the North China Craton.Chuzhou volcanic rocks are located on the eastern boundary of the the northern segment of the Zhangbaling uplift, were erupted during the Early Cretaceous, the eruption ages varied from 132Ma to 116Ma with a long period of 16Ma. 132Ma is the earliest eruption age of the cal-alkaline magma within the Tan-Lu fault zone, and this age should represent the beginning of the lithosphere extensive movement beneath the fault zone. These volcanic rocks are metaluminous and silicon-saturation, belonging to intermediate-acidic high-K cal-alkaline series, and notably rich in LILE and LREE, depleted in HREE and HFSE, and rich in Sr-Nd-Pb isotope component. Geochemical characteristics show that the earlier (about 132Ma) and the later (123～116Ma) rocks were characterized by common cal-alkaline volcanic rocks, medium-term with eruption age (129～125Ma) would resemble in geochemical characteristics of adakitic rocks. The geochemical data indicate a magma source of the North China craton. Early magma reflects a mixing between the EMI-type mantle and ancient lower crust of the NCC. Medium-term magma is a partial melting and mixture of the underplating mantle-derived components mantle and ancient lower crust. The late magma are mostly ancient lower crust components. From early to late, ancient crustal components in volcanic rocks gradually increased, and magma source became shallower and shallower, which implied Chuzhou volcanic rocks were originated from the transitional zone between the crust and mantle. Characteristics of geochemistry and magma evolvement could not support lithosphere whole detachment model, however magma should be formed under this setting of lithosphere thinning gradually from top down. Compared with the geochronology data of the contemporary volcanic rocks from the inner North China craton, the volcanic activities on the Tan-Lu fault zone began earlier and have a comparatively long history. Therefore, Chuzhou volcanic rocks indicated an intensive degree and a long period of the lithospheric thinning within the fault zone accompanying with these characteristics of higher degree melting and scale. More intensive interaction between the crust and mantle, and shallower depth of magma source, further demonstrated the Tan-Lu fault zone may be an extensive lithosphere thinning zone in eastern North China Craton.Chaohu intrusions were obtained from the southern segment of the Zhangbaling uplift along the Tan-Lu fault zone. Five samples show an emplacment ages range of 127～103Ma with 24Ma emplacement period. These intrusions are aluminous and silicon-saturation, a set of transitional magmatic rocks from metaluminous to peraluminous characterized by rich in alkali, potassium. Early and middle-early period intrusions are rich in LILE and LREE, depleted in HREE, and no or positive Eu anomaly, belonging to a set of adakite-like monzonitic granite, in contrast, late and middle-late A-type granites characterized by rich in LILE, and showing negative Ba, Sr, Eu anomalies and lower REE with more weakly fractional degree of LREE/HREE. The earlier adakite should represent the beginning of the lithosphere extensive movement, however the later A-type granites indicate the intensive period of the lithosphere extensive movement. Chaohu intrusions magmas also derived from ancient NNC, particularly ancient lower crust of the NCC. Earlier magmas contained few enriched mantle endmember components, later magmas came from shallower crust source. Geochemical characteristics indicate these intrusions did not suffer from the middle-upper crustal contamination during magma ascent, while the mother magma area experienced different degree partial melting, and fractional crystallization of different rock forming minerals and accessory minerals, such as zircon, apatite and allanite. With different magma emplacement age, residues of magma source are obviously different. Namely, from early to late, residue of magma source are transition from garnet to plagioclase, then to hornblende, accompanying with shallower and shallower magma source. The evolutive disciplines imply deep dynamic process of magmatic activities: continual thinning of lithosphere from NNC bottom led to constantly uplifting of the asthenosphere surface and persistently ascending of isotherm plane within the lithosphere, chemical action became gradually strong, thus appeared the partial melting of the lower crustal source at different depth. Compared with inner craton, the Tan-Lu fault zone show more intensive lithosphere extensive degree, shallower magma source, stronger evolution, higher partial melting extent, more permanent magmatic activities and complicated history. A series of evidences demonstrated the Tan-Lu fault zone is an extensive thinning zone during lithosphere thinning.Lujiang volcanic rocks lies on the most southeastern segment of the Tan-Lu fault zone. The eruption ages ranged from 125Ma to 93Ma, which belong to the middle-late stage of the early Cretaceous to early stage of the late Cretaceous, indicating a 32Ma history of this volcanism. These rocks are rich in alkali and potassium, and relatively depleted in MgO, TiO₂, with enriched Sr-Nd-Pb compositions, belong to cal-alkaline to alkaline series, which having a characteristics of bimodal volcanism. Samples of basalts and trachytes show enriched in LILE, LREE, depleted in HFSE, no negative Eu anomaly. In contrast, rhyolites display negative Ba, Sr, P and Ti anomalies, and obviously negative Eu anomaly. Geochemical data indicate that magma come from four different types of magma source. Earlier basic rocks (125Ma) represent primary nature of NNC lithospheric mantle source accompanying with crustal source. Rhyolites (120Ma) of them come from the middle-lower crust of North China, in addition, trachytes (100Ma) is a mixing of lower crust North China and lithospheric mantle source. Later basic rocks (93Ma) show NNC lithospheric mantle source accompanying with the lower crustal source and asthenospheric component. Geochemic investigations can not support lithospheric mantle and lower crustal detachment model, the magma evolution should be ascribed to the increasing of lithospheric heat flow followed sustaining thinning of its lower part, the asthenosphere upwelling and lithosphere extension also play a key factors for the magmatism. In addition, Some geological, geochemical and geophysical features suggest a more intensive extensive setting, which indicate a more intensive lithospheric thinning beneath this fault zone, the lithospheric thinning may be trigged by the activity of the fault zone, according to results of Chuzhou volcanic rocks and Chaohu intrusions.Investigations demonstrate that although the late Mesozoic magamatic rocks show different rock-type and geochemical characteristics, their magma sources belong to the NNC with a mixing nature of the crust and mantle. Combined with previous data, different segment magmatic activities in the Tan-Lu fault zone have a relatively long duration and a later terminal time, and indicate an intensively extensional tectonic setting. Obvious magmatic evolvement feature represents incessant changes of magma source component and temperature-pressure environment. All evidences may explain lithosphere gradually thinning from the top down, though they could not support whole lithosphere delamination model. A series of evidences demonstrate that the Tan-Lu fault zone is an intensive thinning zone, its occurrence and activities provides an inducing and promoting role during the lithosphere thinning according to geophysical phenomena nowadays, and plays an important role in the destruction of the NCC.