Geochemical Studies On Petrogenesis Of Mesozoic Adakitic Rocks In Central-eastern China And High-temperature Magnesium Isotope Fractionation

One objective of this dissertation is to constrain the petrogenesis and explore geological implications of adakitic rocks through a combined chronoglocial and geochemical study of Mesozoic adakitic rocks from four different regions in eastern-central China. The major research subjects include (1) chronological and geochemical investigations of low-Mg adakitic rocks and associated normal granites and mafic dyke in the Bengbu area, to study the processes and mechanisms of lithospheric thinning of the North China Craton (NCC) and generation of intraplate EMI-type basaltic magmas; (2) chronological and geochemical studies of high-Mg adakitic rocks along the south Tan-Lu fault zone (STLF), to explore the potential role of movement of Tan-Lu fault in triggering delamination of the dense LCC; (3) comprehensive geochemical comparisons between high-Mg adakites from the STLF and the Lower Yangtze River Belt (LYRB), to elucidate the petrogenesis of the LYRB adakites and explore their implications for Cu-Au mineralization; (4) geochemical studies of Xu-Huai high-Mg adakitic rocks and entrained mafic xenoliths as well as the Wuhe group metamorphic rocks in the Bengbu area, to constrain generation of the adakitic rocks and origins of the highly radiogenic Pb isotopic compositions of the ancient lower continental crust (LCC) in the Southeastern NCC.The other objective is to study high-temperature inter-mineral Mg isotope fractionation during igneous differentiation. The major subjects include (5) studies of granitoids and their associated mafic minerals, to investigate the behavior of Mg isotopes during granite differentiation at high temperatures and explore the potential application of Mg isotopes to trace granite source; (6) systematic studies of peridotite xenoiths formed over a wide temperature range of 300℃and their associated mafic minerals, to investigate inter-mineral Mg isotope fractionations among peridotite xenoliths; and (7) analysis of Mg isotopic compositions of adakitic rocks from central-eastern China, to explore if Mg isotopes can be used to discriminate adakites with different origins and discriminate high-Mg adakites from low-Mg adakites.The main results related to this thesis are given in details in the following.1. Low-Mg adakitic rocks from the Bengbu area in the North China Craton: Implication for lithospheric thinningFelsic (granodiorite and granite) and mafic rocks widely occur in the Bengbu area. The granodiorites have zircon U-Pb ages of 118-115Ma, significantly older than zircon U-Pb age of the granites (112Ma). The granodiorites also contain～2.5 and～1.8 Ga inherited zircons. Compared to the granites, the granodiorites have higher Sr contents, lower Y contents, coupledly higher Sr/Y, (Dy/Yb)N and (La/Yb)N, which are similar to low-Mg adakitic rocks (Mg#<48). In addition, the granodiorites have high initial 87Sr/86Sr (0.70752 to 0.70926), lowεNd(t) (21.8 to-18.5), low radiogenic Pb (e.g.,206Pb/204Pb(t)< 17.0), as well as the mantle-likeδ18Mg value. These features are typical of the ancient LCC. The granodiorites were likely derived from partial melting of the thickened LCC of the NCC, while the normal granites were derived from a shallower crustal source. Therefore, there was still a thick LCC in the southeastern NCC before-115Ma, but the thick LCC was suddenly removed between 115 and 112Ma.Besides, the thesis studied diabase dykes in the Bengbu area. The diabases have Ar-Ar age of 112Ma that is younger than the granodiorites but similar to the granites. The Bengbu diabases are characterized by enriched light Rare Earth Elements (e.g., LaN/YbN>10), depleted Nb, Ta and Ti, and a typical EMI-like Sr-Nd-Pb isotopic signature (initial 87Sr/86Sr=0.70641 to 0.70816,εNd(t)=-6.1 to-5.8,206Pb/204Pb= 16.78 to 17.26), indicating an important contribution of the ancient LCC in their mantle source. These features, together with low Ce/Pb and Nb/U and high Ba/Nb and La/Nb, suggest derivation of the Bengbu diabases from partial melting of upwelling asthenosphere mantle, which was hybridized by the delaminated ancient LCC. However, Pb isotopes of the Bengbu diabases are different from the early Cretaceous mafic rocks in the Dabie orogen due to their lower 208Pb/204Pb at for a given 206Pb/204Pb, and instead, they are similar to Mesozoic mafic rocks from the NCC. In particular, the Pb isotopic features are similar to those of the Bengbu low-Mg adakites. Based on spatial-temporal distributions of low-Mg adakitic rocks, normal granites and mafic dykes in the Bengbu areas, together with previously published chronological data, we propose that partial melting of the pre-existing thickened LCC in the Bengbu area at～130-115Ma might have weakned the LCC and left a denser residue, which was subsequently delaminated into the mantle at some point between 115-112Ma, resulting in removal of the thick lithospheric keel. The lithospheric delamination induced mantle upwelling and melting, in conjunction with extensive crustal melting, to produce basaltic and granitic magmas at～112Ma. In addition, we propose that previous melting of the LCC is an important mechanism that weakened the LCC and caused gravitational instability of the LCC. Foundering of the dense LCC into the mantle is likely a crucial way to generate intraplate basaltic magmatism with a typical LCC affinity.2. High-Mg adakitic rocks from the South Tan-Lu fault zone: Implications for foundering mechanism of the lower continental crustWe newly found four high-Mg adakitic plutons from the South part of the Tan-Lu fault zone (STLF), including Xiaolizhuang, Damaochun, Fangjiangzhuang and Qiaotouji. These plutons were formed at 125,128,129 and 131Ma respectively, and become younger with the distance to the Tan-Lu fault zone. They have high Sr (743-1040ppm) and Sr/Y (43-75), low Y (10.1-18.1ppm) and Yb (0.90-1.48ppm), high Mg# (50-67), MgO (2.0-6.1wt.%) and K20/Na20 (0.8-1.0). The initial Sr-Nd-Pb isotopic compositions show an ancient LCC affinity, with 87Sr/86Sr(i)=0.70569-0.70696,εNd(t)=-17.4 to-11.4,206Pb/204Pb(i)=16.258-16.384,207Pb/204Pb(i)= 15.343-15.398,208Pb/204Pb(i)=36.564-36.900.The geochemical features of the STLF high-Mg adakitic rocks suggest that they were likely derived from partial melting of the delaminated lower continental crust, followed by interaction with the mantle. The four high-Mg adakitic plutons reported here, along with three early Cretaceous high-Mg adakitic plutons from the Dabie orogen reported in previous studies, consistute a high-Mg adakitic rock belt that was closely adjacent to the Tan-Lu fault. This strongly suggests that the movement of the Tan-Lu fault during early Cretaceous might play an important role in triggering delamination of the eclogitic LCC along the fault system. This is further supported by the geographic distribution and age variation among the STLF high-Mg adakites identified here.3. High-Mg adakitic rocks from the Lower Yangtze River Belt:Implications for Cu-Au mineralizationThe Lower Yangtze River Belt (LYRB) in central-eastern China is one of important metallogenic belts in China. The host rocks are generally adakitic rocks. Previous studies concluded that the LYRB adakites were derived from partial melting of delaminated LCC, and proposed that delaminated LCC-derived adakitic rocks can also generate Cu-Au mineralization. This thesis studied adakitic rocks from two representative regions in the LYRB, collected all published data for adakitic rocks from the LYRB, and presented a comprehensive geochemical comparison between the LYRB adakites and the ore-barren high-Mg adakitic rocks from the STLF. We found distinct differences in geochemical compositions between these two groups of adakitic rocks:(1) the LYRB adakites are enriched in Na and Al, while the STLF adakites are relatively enriched in K and depleted in Al; (2) At a given SiO2, the LYRB adakites have MgO contents and Mg# similar to slab-derived adakites in subduction zones, while the STLF adakites display even higher MgO and Mg#; (3) The STLF adakites have high Sr/Y and (La/Yb)N, which are positively correlated, while the LYRB adakites show lower. (La/Yb)N but higher Sr/Y ratios; (4) The LYRB adakites have higher low Sr/La and Ce/Pb than the STLF adakites, the latter are consistent with the LCC and the former are close to the MORB; (5) The LYRB adakites have EM2-like Sr-Nd isotopic compositions indicating sediment involvement in source, while the STLF have Sr-Nd isotopic composition of typical LCC; (6) The LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with 206Pb/204Pb(t) up to 18.8, which are clearly distinct from the STLF adakites with low radiogenic Pb (206Pb/204Pb(t)<16.4); and (7) the LYRB adakites have variable and heavy oxygen isotopic compositions in support of sediment involvement, while the STLF adakites have oxygen isotopic compositions close to the mantle value. Based on these differences, we propose that the LYRB adakites were derived from partial melting of subducted oceanic crust, not supporting an origin from delaminated LCC.It is generally accepted that the subduction zones contain sediments and thus have high oxygen fugacity. In contrast, the LCC contains much less water and thus have low oxygen fugacity. Therefore, although partial melts of the delaminated LCC might have interacted with the mantle peridotites, but they were unable to oxidate mantle sulfides and hence failed to generate significant mineralization. In contrast, reaction of slab-derived melts in subduction zones with the mantle could decompose the sulfides and subsequently produce Cu-Au mineralization. This difference well explains the contrasting Cu-Au mineralization between the STLF and the LYRB high-Mg adakites. We analyzed in-situ trace elemental compositions of magmatic zircons from the two groups of high-Mg adakites, and found that the LYRB adakites have much higher Ce3+/Ce3+and Eu3+/Eu2+ratios than the LYRB adakites, demonstrating that initial magmas of the former indeed have much higher oxygen fagucities.4.Origin of high-Mg adakitic rocks from Xu-Huai area in the North China Craton and evolution of the lower continental crust in this areaPrevious studies proposed that the Xu-Huai adakitic rocks in the southerastern NCC were derived from partial melting of the delaminated LCC of the NCC, and the entrained xenoliths represent source rocks of the host rocks. However, we found that although the Xu-Huai adakitic rocks have high Sr/Y and (Dy/Yb)N, but they are characterized by relatively low (La/Yb)N. In addition, they are enriched in Na and depleted in K. These features are different from those of adakitic rocks derived from partial melting of delamainted LCC, but similar to adakites derived from partial melting of subducted oceanic slab. Moreover, initial Sr-Nd-Pb isotopic compositions of the Xu-Huai adakitic rocks are different from those of the entrained mafic xenoliths: Sr and Nd isotopes of the adakitic rocks show a positive correlation with two endmembers towards the EM-2 and ancient LCC (EM-1) respectively, while the xenoliths display a vertical trend from bulk Earth to EM-1. Although both the Xu-Huai adakitic rocks and the entrained mafic xenoliths have higher radiogenic Pb, they are different in Pb isotopic compositions. The Pb isotope data of the Xu-Huai xenoliths and garnet amphibolites as well as marbles from the Wuhe group (the Archean basement rocks in the Bengbu uplift) defined a perfect Pb-Pb isochron in 207Pb/204/Pb-206Pb/204Pb diagram, while Pb isotopes of the adakitic rocks are similar to MORB and fall below the Pb-Pb isochron defined by the Xu-Huai xenoliths and the Wuhe group metamorphic rocks. These observations indicate that the Xu-Huai adakitic rocks were not derived from partial melting of the ancient LCC of the NCC represented by the xenoliths, as proposed by Xu et al. (2006). In addition, it is difficult to imagine that the delaminated LCC-derived magmas could carry their source rocks (eclogite) to the surface. Also, most of the Xu-Huai xenoliths underwent amphibolite facies retrograde metamorphism, which appears not a result of fast magma ascending. Based on these observations, we propose that the Xu-Huai high-Mg adakitic rocks were likely derived from partial melting of subducted oceanic crust, followed by extensive contamination with the LCC rocks during magma intrusions.Mafic garnet-pyroxenite/garnet-amphibolite xenoliths from the Xu-Huai area and garnet-pyroxene amphibolites as well as the marble wall rocks from the Bengbu area consistute a Pb-Pb isochron with age of～1.95Ga, which is consistent with previously reported high-pressure granulite facies metamorphism time based on zircon U-Pb dating. This implies large-scale Pb isotopic homogenization of the Precambrian basement LCC in the southeastern margin of the NCC caused by the early Proterozoic granulite-facies metamorphism. Substantial surface materials involved in the Wuhe group, such as marbles and graywackes, are likely the main source of the highly radiogenic Pb. The results also imply that using the ancient LCC to balance the terrestrial Pb needs take into account the ancient LCC which has an igneous precursor but are characterized by highly radiogenic Pb.5. Magnesium isotopic studies of granitoids and mafic mineralsThis dissertation reports the first systematic study of Mg isotopic compositions for a suite of granitoids and their associated mafic minerals. Although these granitoids formed through different degrees of partial melting and fractional crystallization with large variations in elemental and mineral compositions, theirδ26Mg values vary from-0.26 to -0.14%o and are indistinguishable within the analytical precision (±0.07‰; 2SD). This suggests limited Mg isotope fractionation during granite differentiation.Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic compositions with△26MgHbl-Bt=-0.06±0.08‰(2SD), further indicating that fractional crystallization of either biotite or hornblende did not significantly change Mg isotopic composition of the host magma, consistent with conclusions based on whole-rock analysis. Therefore, Mg isotopic compositions of granitoids can be used to trace their sources. This provides basis for applying Mg isotopic compositions of adakites to trace their magma sources and thus, magma genesis. Besides, the limited inter-mineral Mg isotope fractionation between coexisting hornblende and biotite agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation at high temperatures. 6. High-temperature inter-mineral Mg isotope fractionation in mantle xenolithsPrevious studies have reported large amounts of Mg isotopic data for mantle minerals (Opx, Cpx, olivine and spinel), but whether there are detectable Mg isotope fractionations between these minerals and whether the isotope fractionation are in equilibrium or not are highly debated. In order to solve these debates, this dissertation measured magnesium isotopic compositions of coexisting Opx, Cpx, olivine and spinel from a suite of mantle xenoliths, which span a wide temperature range (>300℃), and for the first time analyzed phlogopite (Phl).The results show that coexisting Opx and 01 have constant Mg isotopic compositions over the overall temperature range, which are indistinguishable within the analytical precision. In contrast, Mg isotopic compositions of Cpx and Phl are significantly different from coexisting ol, with Cpx and Phl slightly heavier than coexisting 01. In addition, isotope fractionation between Cpx and 01 are related to temperatures, implying equilibrium isotope fractionation. In particular, spinels are much heavier in Mg isotopic composition compared to coexisting silicates, with△26Mgspl-Ol ranging from+0.25 to+0.55%o (n=10). The degree of inter-mineral Mg isotope fractionation among these mantle minerals agrees with theoretical predictions, suggesting that inter-mineral Mg isotope fractionation is primarily controlled by the Mg-0 bond strength, with stronger bonds favoring heavy Mg isotopes. Magnesium is in tetrahedral coordination in spinels and in octahedral coordination in other mantle silicates. Therefore, the theoretical predications confirm that spinels are much heavier than coexisting silicates. Our results demonstrate that the large high-temperature equilibrium Spl-Ol Mg isotope fractionation in peridotite xenoliths can potentially be used as a geothermometry in mantle geochemistry.7. Mg isotopic compositions of adakitic rocks from central-eastern ChinaAfter having demonstrated that there is limited Mg isotope fractionation during granite differentiation, this thesis analyzed Mg isotopic compositions of adakitic rocks that have various Mg#'s and origins from central-eastern China, including low-Mg adakitic rocks from the Dabie orogen, high-Mg adakitic rocks from the LYRB, the STLF and the Xu-Huai area. We found that all of these high-Mg adakitic rocks except one Xu-Huai sample have very homogeneous Mg isotopic composition, with averageδ26Mg=-0.25±0.05‰(2SD), which is indistinguishable from the mantle value (δ26Mg=-0.25±0.05‰; 2SD). These observations suggest that the thickened lower crust in the Dabie mountain root is mainly derived from the mantle by magma under-plating, and they also indicate that although MgO and Mg# of adakitic magmas during interaction with the mantle prominently increase, their Mg isotopic compositions did not significantly change. This reflects that the mafic LCC and MORB themselves have Mg isotopic compositions similar to the mantle, so that the melt-mantle interaction did not significantly modify Mg isotopic composition of the initial LCC/MORB melts. Therefore, Mg isotopes of adakites can not be used to identify origins of adakites and discriminate high-Mg adakites from low-Mg ones. In addition, although Sr-Nd-Pb-O isotopic compositions of the LYRB adakites imply sediment involvement in magma sources, their homogeneous and mantle-like Mg isotopic compositions suggest fast Mg isotope exchange equilibrium between adakitic rocks and the mantle peridotites. The Xu-Huai sample that has relatively light Mg isotopic composition has high CO2 content, suggesting involvement of recycling carbonatite in the magma source, consistent with its high radiogenic Pb signature.