Elemental Geochemistry And Calcium Isotope Studies Of Early Cretaceous Magmatic Rocks In The Lower Yangtze River Belt

We focus on the lower Yangtze River belt(LYRB).We studied the adakites and A-type granites in the study area by using the method of element geochemistry and Ca isotope.We first evaluated the effect of radiogenic ⁴⁰Ca(R⁴⁰Ca)resulting from ⁴⁰K decay on stable Ca isotopic data onto the K-Ca isotopic system.A simple and effective algorithm is established to correct R⁴⁰Ca.Then,a model weighted average estimation algorithm is presented for tracking the Ca isotopic composition of the upper continental crust(UCC).Finally,in this paper,the fractionation mechanism of Ca isotopes during magmatic evolution is discussed.The geological evolution history of Late Mesozoic in the LYRB is inversed.In the rocks with high K/Ca value and older formation age,the ⁴⁰Ca formed by?-decayed of the ⁴⁰K will accumulate in large quantities,resulting in the increase of?^44/40Ca value eventually.It will bring trouble to data interpretation.In this paper,based on the calculation of double spike,R⁴⁰Ca can be accurately monitored and deducted by adjusting the calculation parameters and algorithm.The results show that there is a linear correlation between R⁴⁰Ca and the real?^44/40Ca of sample of the iteration method.Therefore,R⁴⁰Ca can be calculated by the decay law,and then the true?^44/40Ca of the sample can be obtained.The advantage of this method is that it does not need more chemical separation and mass spectrometry to calculate the?^44/42Ca value to calculate the?^44/40Ca of the sample.It is not only convenient and fast,but also improves the experimental efficiency.Early Cretaceous adakites are widely distributed in LYRB,the south Tan-Lu Fault(STLF)and the Dabie orogen in central eastern China.Their genesis,however,remains controversial.In this contribution,we present detailed geochemical and geochronological study on the Guandian pluton in central Anhui Province,eastern China,which has been formerly regarded as a part of the north belt in the LYRB and lately classified in the STLF.The Guandian pluton consists of monzonite and quartz monzonite,and is metaluminous and high-K calc-alkaline according to the chemical composition.The samples show high Si O₂(59.15-62.32%),Al₂O₃(14.51-15.38%),Sr(892-1184 ppm),Sr/Y(57.63-86.32)and low Y(12.65-18.05 ppm),similar to typical geochemical features of adakite.The Guandian adakite also exhibits high K₂O(2.88-3.86%),Mg O(3.89-5.24%)and Mg#(55-60),negative anomalies of high field strength elements(HFSE,e.g.,Nb,Ta and Ti)and positive anomalies of Ba,Pb and Sr.LA-ICP-MS zircon U-Pb dating yielded a weighted average age of 129.2±0.7 Ma.Calculations of zircon Ce⁴⁺/Ce³⁺(6.79-145)and(Eu/Eu*)_N(0.23-0.42)on the basis of in situ zircon trace element analysis indicate that the magma had a lower oxygen fugacity relative to the ore-bearing adakites in the LYRB and Dexing,which is consistent with the fact of ore-barren in the research area.In combination with previous research,we propose that Guandian adakite was formed by partial melting of delaminated lower continental crust triggered by Early Cretaceous ridge subduction of the Pacific and Izanagi plates.A-type granites are usually exposed at extensional tectonic settings,and play an important role in understanding of Earth's crustal evolution.However,the genesis of A-type granite is still controversial.Here we report high-precision Ca isotopic compositions of well-characterized A-type granites from the LYRB,eastern China.They show similar ages and homogeneous Sr,Nd and Hf isotopic compositions,indicating that they originated from the same magma source.The?^44/40Ca ratios of the A-type granites range from 0.51‰to 0.99‰,and are negatively correlated with Ca O,Sr and Ba contents and Eu anomaly(defined as Eu*).The results show that the Ca isotope fractionation is caused by differentiation of plagioclase.The modeling results show that?70%of plagioclase has been crystallized in most of the samples,which is supported by mineralogical observations.Sample of the lowest evolution represented the composition of the source region.Therefore,the lightest?^44/40Ca(0.51‰)of the A-type granites implies the existence of a potential light Ca isotopic source.Combined with the geodynamics background of the LYRB,we propose that A-type granites of LYRB were formed by partial melting of enriched mantle metasomatized by subducted sediments.In a strong extensional tectonic setting,magma rises along the fault zone and intrudes into the crust to form A-type granite.During the magmatic evolution.it through a slightly AFC process and fractional crystallization of plagioclase.