The Petrogenetic Link Between Volcanic And Plutonic Rocks Of The Cretaceous Caldera Complexes In The Coastal Area Of The Zhejiang And Fujian Provinces

Silicic magmatic rocks are important and characteristic components of the continental crust,which may record the formation,differentiation and reworking of the continental crust and the interaction between the crust and mantle.The silicic magmas could be produced by partial melting of crustal assemblages or fractional crystallization of mantle-derived mafic magma.They may form either below the surface as intrusive(plutonic)rocks or on the surface as extrusive(volcanic)rocks.The genetic relationship between silicic plutonic and volcanic rocks is one of the key questions to understand the origin and evolution of silicic magma systems.The caldera systems commonly well preserve the volcanic rocks and associated shallow plutons,which may provide unique constraints on the evolution of the silicic magmatic system and the petrogenetic link between volcanic rocks and plutonic rocks.A large-scale Cretaceous volcanic-plutonic complex belt was developed in the coastal area of SE China,containing several volcanic fields,which provides an optimal to study the petrogenetic link between volcanism and associated plutonism and related magmatic processes.In this paper,we conducted petrological,zircon U–Pb chronology,zircon Hf and stable O isotopes along with geochemical data(major elements,trace elements and Sr-Nd isotopes)of coexisting volcanic and plutonic rocks from the Yandangshan and Yunshan caldera complexes in Zhejiang and Fujian provinces respectively,to constrains the magmatic origin and evolution processes and evaluate the petrogenetic link between volcanism and plutonism in the caldera complexes.The Yandangshan caldera complex is composed of a series of rhyolitic welded tuff and rhyolite and coexisting subvolcanic intrusion of porphyritic quartz syenite.The SHRIMP and LA-ICP-MS zircon U–Pb dating resultsindicated that the crystallization of the rhyolitic extrusives and subvolcanic intrusions was contemporaneous within analytical errors and in a very short period(104–98 Ma).Geochemically,the volcanic rocks are characterized by high Rb/Sr and Rb/Ba ratios and depletions in Ba,Sr,P,Eu and Ti,while the shallow plutons show high K,Ba,Aland Fe and low Rb/Sr and Rb/Ba ratios with insignificant negative Eu anomalies.The volcanic and plutonic rocks have comparable whole-rock Sr and Nd isotopic compositions [(87Sr/86Sr)i=0.7084–0.7090;?Nd(t)=-7.8 to-6.5],and have a similar range of zircon Hf–O isotopic compositions [?Hf(t)=-10.0 to+1.5;TDMC=2.10–1.23 Ga;?18O mainly = 4.5 to 6.0‰].The above features indicate that they have a same magma source,which were formed by reworking of Paleoproterozoic lower crusts in the eastern Cathaysia block.The volcanic rocks in the Yunshan caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton.SHRIMP zircon U–Pb geochronology of both volcanic and plutonic rocks yields almost identical crystallization ages ranging from 95.6 to 93.1 Ma.The peraluminous and peralkaline rhyolites show negative anomalies of Sr,P,Ti and Ba and to a lesser extent negative Nb and Ta anomalies,along with positive Rb anomalies and ‘seagull-like' rare earth element(REE)patterns with strong negative Eu anomalies and low(La/Yb)N ratios.The porphyritic quartz monzonite displays minor negative Rb,Nb,Ta,Sr,P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high(La/Yb)N ratios and lack significant Eu anomalies.The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent ?Nd(t)of-3.7 to-2.2 and display zircon ?Hf(t)values of-2.1 to +3.7.They further have similar,mantle-like,zircon oxygen isotopic compositions(?18OVSMOW mainly = 4.63 to 5.76‰).The parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge.The geochemical characteristics of the peralkaline rhyolites show that they have experienced a high degree of differentiationand significant volatile–magma interactions.The volcanic rocks of both the Yandangshan and Yunshan caldera complexes display geochemical characteristics of highly evolved magmas,whereas,the coexisting plutonic rocks show a complementary geochemical signature with the volcanic rocks.We thus suggest that the volcanic rocks and the plutonic rocks were derived from a common magma source and that the magmas of the volcanic rocks are formed by the extraction of interstitial melt from crystal mushes,while the plutonic rocksrepresent the residual crystal-rich mushes after extraction of the rhyolitic melts with geochemical characteristics of crystal accumulation.Furthermore,the Yunshan rhyolites typically match the geochemical characteristics of ‘hot-dry-reduced' rhyolites indicating that,during the late Cretaceous,the tectonic setting of SE China changed from a compressional environment to an extensional environment,i.e.,from an arc into a back-arc setting.