Petrogenesis of the Mesozoic Tongshanling Granodiorite in Southern Hunan Province, South China: Clues from in-situ Nd Isotopes and Elements of the Titanite

The compositions of the bulk-rocks are commonly modified by hydrothermal alteration or merely represent the magmatic information of snapshot during evolution. Hence, use of the chemical compositions of bulk-rocks makes it difficult to trace the source and evolution of granitic magmas. The petrogenesis of the granitic rock is therefore difficult to decipher. In the face of these difficulties, an alternative new approach is to trace the magmatic source and evolution by in-situ analyzing elements and isotope compositions of accessory minerals in granitic rocks. This new approach can significantly improve the spatial resolution of the magmatic processes. Titanite (CaTiSiO₅) is a common accessory mineral of granitic rocks and contains a large amount of elements, including Al, Fe, Nb, Ta, Zr, Cr, V, Sn in the Ti site, and rare earth elements (REEs) Y, Na, Mn, Pb, U, Th, Sr and Ba in the Ca site. REEs and high field strength elements (HFSEs, i.e., Nb, Ta, Zr, Th and U) in titanite are sensitive to the changes of temperature, pressure, oxygen fugacity (f_O2), water fugacity and melt composition. Additionally, titanite has high Nd concentration and low Sm/Nd ratio and is suitable for in-situ Nd isotope analyses. Therefore, titanite is an ideal accessory mineral to be used to investigate the magmatic source and evolution of granitic rocks.

To trace the magmatic source and evolution, and to decipher the petrogenesis of the granitic rock.

Titanite from mafic microgranular enclave (MME) and hosted granodiorite of the Tongshanling granitic pluton were checked by transmission light and back-scattered electron (BSE). BSE images were performed at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) in Guiyang, on carbon-coated, polished epoxy blocks using JSM-7800F field emission scanning electron microscopy (SEM) operated at 20kV accelerating voltage and a beam current of 10nA. Their in-situ major element compositions were analysed by electron probe microanalyzer (EPMA) at IGCAS. An accelerating voltage of 25kV and a probe current of 10nA were applied. Well-characterized Kaersutite (Na, K, Mg, Al, Si, Ca, Mn, and Fe), apatite (F), and rutile (Ti) were used as standards. The trace elements of titanite were analysed in-situ by an Agilent 7900 ICPMS equipped with GeoLasPro 193nm ArF laser ablation system (LA-ICP-MS) at IGCAS. Analytical conditions were as follows: a fluence of 5J/cm², at a repetition rate of 5Hz and laser spot of 44μm. Ca (determined by EPMA) was chosen as the internal standard and the reference glasses NIST610 and NIST612 were used to calibrate relative element sensitivities. In-situ Nd isotope of titanite were analyzed by a Nu Plasma Ⅲ multi-collector (MC) equipped with RESOlution-155 ArF 193nm laser ablation system (LA-MC-ICP-MS) at IGCAS. Titanite was ablated in a mixture of helium (350mL/min) and nitrogen (2mL/min) atmosphere using the following parameters: 30s baseline time, 40s ablation time, 72μm spot size, 6Hz repetition rate and 6J/cm² energy density. The interference of ¹⁴⁴Sm on ¹⁴⁴Nd was derived from the ¹⁴⁷Sm intensity with a natural ¹⁴⁴Sm/¹⁴⁷Sm ratio of 0.205484. The mass bias factor of Sm was calculated from the measured isotopic ratio of ¹⁴⁴Sm/¹⁴⁹Sm and its true value 1.08680. The mass bias of ¹⁴³Nd/¹⁴⁴Nd was normalized to ¹⁴⁶Nd/¹⁴⁴Nd=0.7129 with an exponential law. The reference materials MAD, Otter Lake, LAP and SAP were chosen as the external standards.

Titanite grains in the MME, and host granodiorite of the Tongshanling granitic pluton have similar major element compositions. All titanite are characterized by high SiO₂ (31.0% to 31.7%), CaO (29.0%-30.0%), TiO₂ (30.6%-38.2%), and low Al₂O₃ (1.81%-5.61%), FeO (0.184%-0.606%), F (0.48%-1.84%) contents. The MnO concentrations vary between 0.024% and 0.066%. The MgO concentrations range from 0.001% to 0.031%. Compositional zoning among single titanite grains were not observed. The crystallo-chemical formulae were calculated on the basis of 5 oxygen atoms. The calculated results indicate that the Al+Fe (apfu) is negatively correlated with Ti (apfu) in all titanite grains. The analysed titanite grains have high rare earth element (REE) contents (67-1498μg/g). The REE were incorporated into titanite lattice by substituting for Ti and Ca site with Al and Fe and the substituted mechanism is (Al, Fe³⁺)+REE=Ti⁴⁺+O^2-. On the REE patterns, the titanite from granodiorite has REE contents higher than that from MME and shows weak positive or negative Eu anomaly (Eu/Eu^* from 0.62 to 1.39). On the contrary, titanite from MME is characterized by Eu positive anomaly (Eu/Eu^* from 1.13 to 3.94). These titanite contain high content of HFSEs such as Zr (11.1-536μg/g), Hf (0.639-21μg/g), Nb (306-1489μg/g) and Ta (24.7-109.5μg/g). The variation of Zr/Hf, Nb/Ta and Y/Ho ratios of titanite grains range from 21.0 to 31.5, 10.4 to 13.9 and 27.4 to 35.0, respectively. These trace element ratios are consistent with those of normal crust and are not fractionated. Therefore, the trace elements of titanite were completely controlled by ion radius and charge and not affected by late hydrothermal alteration. Titanite Zr thermometer shows that the temperature of titanite formation is between 762℃ and 963℃. Titanite from MME has homogenous Nd isotope compositions. Their present ¹⁴⁴Nd/¹⁴³Nd ranges from 0.512321 to 0.512675, corresponding to ε_Nd(t) value from -3.5 to -8.9 with an average of -7.2±2.4 (N=6). Titanite from granodiorite overall have ¹⁴⁴Nd/¹⁴³Nd ratio ranging from 0.512269 to 0.513399. Their time-corrected initial ε_Nd(t) value vary between -5.4 and -9.9 with an average of -6.9±2.4 (N=5). All titanite grains have negative initial Nd isotopic compositions, which is consistent with the evolution trend of Nd isotopes of the middle-lower continental crust of South China.

Titanite grains in the MME and host granodiorite of the Tongshanling granitic pluton show little or no intra-grain concentric zoning in BSE images and display similar element and isotopic geochemical characteristics. Crystal chemical exerts a first-order control on elemental compositions of titanite. Titanite survived during hydrothermal alteration and faithfully recorded the information of granitic melts. The granitic melts of the Tongshanling are characterized by high temperature and oxygen fugacity. Granodiorites from the Tongshanling pluton were probably formed by the amphibole-dehydration melting of a mafic source in the middle-lower crust beneath South China.