Geochronological And Petrological Study Of Quartz Diorites In The Rokko Mountains Of Southwest Japan: Implication For Tectonic Evolution

The Rokko Mountains located in the north of Kobe city,Southwest Japan,mainly consist of the late Cretaceous granitic rocks.The Dobashi quartz diorite occurs as an xenolith in the Rokko granite.In this paper,biotite and hornblede minerals from the Dobashi quartz diorite samples are analyzed for the ⁴⁰Ar-³⁹Ar dating,and the hornblende ⁴⁰Ar-³⁹Ar plateau age of 82.3±1.6Ma was obtained,representing that of the emplacement of the Dobashi quartz diorite.The biotite ⁴⁰Ar-³⁹Ar plateau age of 63.3±1.7 Ma was obtained,indicating an intensively late thermal event.Combined with other published K-Ar age data,the ⁴⁰Ar-³⁹Ar ages indicate that multiple tectono—magmatic events occurred in Rokko Mountains.Mineral chemically,Zoning textures well developed in plagioclase phenocrysts of granites and Dobashi quartz diorites,Neutral to acidic from core to rim;in feldspar,the Sr content decreases with an increase in SiO₂ content,the feldspar has more BaO compared to plagioclase,the SrO content increase with an increase in BaO content,The biotites and hornblende in the fine-grained facies granites and Dobashi quartz diorites are rich in Fe contents and poor in Mg contents,belonging to iron-amphibole and iron-biotite respectively. The F and Cl contents in biotites and hornblende of quartz diorite of inclusions are higher than that in quartz diorites and granites,the F and Cl content in biotites and hornblende increase with an decrease in Mg^# content,indicating F and Cl content enrich in late magmatism.Analysis of trace and rare earth elements from the granitic rocks,show that the Rokko fine-grained facies granites have higher Rb,Nb and Y contents,and lower Sr contents compared to the medium—to coarse—grained facies granites.The Ba and Sr content in the Rokko granite decreases with an increase in Rb content.The granites exhibit rich is LREE,Eu /Eu* between 0.62 and 0.9,indicating certain negative Eu anomalies in C1 chondrite-normalized REE patterns.The C1 chondrite-normalized spider diagrams to the C1 Chondrite source show that all granitic rocks from the Rokko Mountains and Southeast of China have the same environment belonging to calc-alkaline series,are enriched in LIL elements and are depleted in HFS elements.Nb,Ta,P,Ti depleted indicated an subduction-related calc-alkaline series of island arc or active continental margin chemical characteristic.It has been extensively proved that the Late Mesozoic granitic igneous rocks in the southeastern coastal area of China have ages from 140 Ma to 100 Ma,which were generated in arc or intra-plate environment,while Rokko granite 83-72 Ma,I-type granite generated in vocalnic arc environment.Combined with other research data and results,it is found that from Cretaceous,as the time goes by,the arc-related volcanic activities migrate from west to east. It is inferred that from Cretaceous,the subduction of Pacific Plate to the Eurasia Plate causes arc-related volcanic activities and the back-arc extension,thinning of the crust,the upwelling of the mantle materials in the southeastern coastal area of China.Basaltic magma from mantle upwells and heats the lower crust,resulting in its partial melting to form granitic magma.We employ a roll-back model of the Pacific Plate to explain the W-E migration of the arc-related activities.