浙东晚白垩世小雄破火山中火山-侵入杂岩的岩石成因

Petrogenesis of Late Cretaceous volcanic-plutonic complex from Xiaoxiong caldera in East Zhejiang

破火山内出露的火山岩与浅成侵入岩为硅质岩浆演化研究提供了一个重要窗口,从而备受关注。小雄破火山内的火山-侵入杂岩是中国东南沿海晚白垩世岩浆活动的典型代表,包括小雄组火山岩(K_(2)x)与两类侵入岩(花岗斑岩、正长斑岩)。本文以小雄火山-侵入杂岩为研究对象,开展了系统的锆石U-Pb年代学、岩石学和地球化学研究,旨在深入探讨破火山内火山岩与侵入岩之间的成因联系和岩浆演化过程。系统的LA-ICP-MS锆石U-Pb年代学研究表明,小雄组火山岩形成于98~88Ma,并具有多期次喷发的特点,可分为下段、中段和上段,年龄分别为98~96Ma(K_(2)x^(1))、95~92Ma(K_(2)x^(2))、~88Ma(K_(2)x^(3))。小雄花岗斑岩形成年龄为90Ma;正长斑岩形成稍晚,约88Ma。与下段流纹质玻屑凝灰岩的Nd-Hf同位素组成[ε_(Nd)(t)=-8.3~-7.2,ε_(Hf)(t)=-11.8~-7.2]相比,中段流纹岩要更为亏损[ε_(Nd)(t)=-5.84~-5.32,ε_(Hf)(t)=-10.1~-0.5]。研究表明,小雄组流纹质火山岩的母岩浆可能起源于发生在深部岩浆房中渐进的壳幔相互作用,中段流纹岩的源区混入了更多的亏损幔源组分。中段流纹岩与花岗斑岩具有相似的Nd-Hf同位素组成,以及“互补”的微量元素地球化学特征,由发生在浅部岩浆房的分离结晶作用和堆晶作用所制约。值得注意的是,正长斑岩与花岗斑岩并不存在直接的成因演化关系,两者应是不同的起源。不同的正长斑岩岩株具有高度一致的结晶年龄、微量元素特征以及Nd-Hf同位素组成,以上特征均表明小雄破火山内的正长斑岩具有相同的起源。正长斑岩母岩浆起源于富集岩石圈地幔的部分熔融,岩浆源区混入了来自亏损的软流圈地幔组分,其地球化学成分变化主要受“普通辉石+磷灰石+钛铁矿”的分离结晶所控制。

The association of volcanic and shallow plutonic rocks in calderas may provide important clues to the geochemical evolution of silicic magma systems,and thus it has received a lot of attention.The Xiaoxiong caldera,mainly composed of volcanic rocks of the Xiaoxiong Formation(K_(2)x)and two types of plutonic rocks(granite-porphyry and syenite-porphyry),is a typical product of Late Cretaceous magmatism in SE China.In this paper,we conducted petrological and geochemical studies,as well as zircon dating and trace element analysis for the coexisting volcanic and plutonic rocks from the Xiaoxiong caldera,aiming to constrain the magmatic origin and evolution processes and evaluate the petrogenetic relationship between the volcanism and plutonism.LA-ICP-MS zircon U-Pb dating results revealed that the Xiaoxiong Formation was formed in 98~88Ma with characteristics of multi-staged eruptions.The ages of the Ⅰ-,Ⅱ-,Ⅲ-cycle volcanic rocks are constrained at 98~96Ma(K_(2)x^(1)),95~92Ma(K_(2)x^(2))and~88Ma(K_(2)x^(3)),respectively.Meanwhile,zircon U-Pb dating provides crystallization ages of ca.90Ma for granite-porphyry and ca.88Ma for syenite-porphyry.However,in terms of Nd-Hf isotopic compositions,the rhyolites from the middle section[ε_(Nd)(t)=-8.3~-7.2,ε_(Hf)(t)=-11.8~-7.2]record increasingly depleted characteristics to the rhyolitic vitric tuffs from the lower section[ε_(Nd)(t)=-5.84~-5.32,ε_(Hf)(t)=-10.1~-0.5],implying a progressive crust-mantle interaction,and more juvenile,Nd-Hf isotopically depleted materials were involved into the source of these rhyolites.The rhyolites from the middle section and the granite-porphyry show“cogenetic”and“complementary”geochemical signatures.Therefore,we suggest that they could be closely linked by fractional crystallization and crystal accumulation in a shallow magma chamber,i.e.,these rhyolites are formed by the extraction of interstitial melt from crystal mush,while the granite-porphyry represents the residual crystal mush.It is worth to note that there is no directly evolutional relationship between granite-porphyry and syenite-porphyry,and they are more likely to have different magmatic origins.In addition,the syenite-porphyries of different stocks in the Xiaoxiong caldera have a highly consistent crystallization ages,trace element characteristics and Nd-Hf isotope compositions,which indicates that they have the same magmatic origins.The syenite-porphyry was generated probably by“pyroxene+apatite+ilmenite”-dominated fractional crystallization from basaltic magma that was produced by magma mixing between melts derived from depleted asthenosphere and subduction-related enriched lithospheric mantle.