Geochemistry Of The Yangyuan Carbonatites And Xenocrysts Therein And Its Implication For The Evolution Of Lower Crust Beneath The Northern North China

Carbonatites are generally thought to be mantle-derived magmatic rocks derived from primary carbon in the mantle or carbon transported into the mantle by subduction.They have the potential to provide direct insights into the deep carbon cycle due to their exceptional concentration of carbon.Deep mantle and subduction zone have been discussed as the source for the carbonatites,however,carbonate sediments stored in the deep continental crust may be another important reservoir which is commonly overlooked.Since formation of the intact North China Craton during the Paleo-Proterozoic,its northern margin evolved into continental margin accompany with break-up of the Columbia supercontinent.Massive of carbonate sediments deposited on the northern North China Craton.Since the Mesozoic,with the thinning of North China Craton,large scale of magmatism occurred in the northern margin,made it a good area for our study.Here,we conducted systematic geochemistry,mineralogy,and geochronology research for the carbonatite and xenocrysts therein from the Yangyuan,North China.We show that the Yangyuan carbonatites are the products of anatectic melting of limestones thrust into the lower continental crust.Moreover,geochemical and geochronological study of the rutile xenocrysts reveal an episodic cooling history of the northern North China Craton from assembly to break-up of the Columbia supercontinent.Different from typical carbonatites,the Yangyuan carbonatites are not enriched in rare earth elements?REEs?,with slight enrichments in light REEs and negative anomalies in Ce and Eu.Limestone-like trace element patterns,radiogenic Sr-Nd-Pb isotopic ratios(Sr: 0.707920–0.710378;Nd: 0.511504–0.512513;²⁰⁶Pb/²⁰⁴Pb = 18.164–18.735,²⁰⁷Pb/²⁰⁴Pb = 15.383–15.680,and 208Pb/²⁰⁴Pb = 38.534–39.161),and heavy oxygen isotopic compositions(?¹⁸O_V-SMOW: +21.3‰ to +22.5‰)show that the carbonatites originate by partial melting of the Phanerozoic limestones.However,their variation show that the melts have experienced assimilation of the lower continental crust.This process was accompanied by CO₂ degassing,documented by trace elements in grossular-rich garnets and declining ?¹³CV-PDB?from-7.08‰ to-9.95‰?,while slightly positive shift of ?¹⁸O is explained by loss of a H₂O-dominated fluid at decreasing temperature.Abundant xenocrysts including zircons,feldspars,and almandine-rich garnets were captured during ascent of the carbonatites.U-Pb ages and Hf isotopes of the zircons indicate their derivation from the basement of the North China Craton.Feldspars can be classified into two groups,with one from Archean basement show unradiogenic Pb isotopes,and the other from the transformed basaltic lower crust show relative radiogenic Pb isotopes.The almandine garnets show enrichments in heavy REEs and Eu negative anomalies,indicate their origin from lower crustal granulites.Therefore,combined with tectonic study,it was proposed that the Phanerozoic limestone precursors were deposited on the North China Craton and thrust to lower crustal depths during the development of the Yinshan-Yanshan fold and thrust belt in the Mesozoic.In the Cenozoic,deeply subducted Paleo-Pacific slab caused asthenospheric upwelling beneath the central zone,resulting in anatectic melting of the limestones in the lower continental crust.This process may have commonly occurred in the convergent margins and contributed to the CO₂ budget in the atmosphere.The Trans-North China Orogen?TNCO;also known as the Central Orogenic Belt?,has been the subject of much attention in an attempt to understand the amalgamation of the North China Craton?NCC?and the assembly of the Columbia supercontinent.However,thermal evolution since that is currently poorly constrained.This study presents U-Pb ages,trace element compositions,and Hf isotopes of rutile xenocrysts carried by the Yangyuan Cenozoic carbonatites.The Zr-in-rutile thermometer indicates formation of the rutiles under granulite facies conditions at 743 to 932 ?.Cr and Nb concentrations in rutiles support a pelitic protolith,which is consistent with 176Hf/177 Hf ratios that are close to zircons from pelitic granulites in the TNCO.U-Pb ages of the rutiles are cooling ages,for which three populations can be clearly delineated.In combination with dating results from other systems,three episodes of cooling can be identified: [1] active cooling after the ¹.85 Ga peak metamorphism,with a cooling rate increasing from 7.9 ?/Ma in the period 1850–1810 Ma to 15.3 ?/Ma at 1810–1792 Ma;[2] active cooling after mantle upwelling at ¹.78 Ga,with cooling rate increasing from 8.7 ?/Ma at 1780–1735 Ma to 17.2 ?/Ma between 1735–1719 Ma;[3] passive cooling after a rifting event at ¹.68 Ga,with cooling rate decreasing from 20.6 ?/Ma at 1680–1673 Ma to 6.9 ?/Ma in the period 1673–1633 Ma.The first two episodes correspond to exhumation-related cooling,while the last episode resulted from the rift-to-drift transition of the North China Craton due to increasing distance from the heat source during break-up of the Columbia supercontinent.