Abstract: Three gabbroic xenoliths have been found for the first time from Early Cretaceous intrusions of quartz monzonitic dioritic to pyroxene monzonitic dioritic composition in Tongling,Anhui Province,China.This paper presents new petrographic observations and microprobe analyses,as well as petrochemical and geochemical analyses on the xenoliths and their hosts,and provides new insight into their formation process and tectonic setting.Two types of gabbroic xenoliths have been identified in these intrusions based on their texture:one with "porphyritic"texture,and the other with gabbro texture.The gabbroic xenoliths are lower in ω(A1203)(13.12％～13.13％),but higher in ω(MgO)(11.28％～11.66％) and alkali co(K20+Na20 =5.50％ ～5.81％).They ale rich in Cr(169.9～186.9 tm／g),Co(19.2 tm／g)and Ni(299.4～356.7 ug／g),and have smaller LREE to HREE ratios(4.43～5.94) with enrichment in Y(22.92～23.56 tc／g).Their initial 87Sr/86Sr ratios range from 0.706 3 to 0.7069,with epsilon Nd(t)= -4.9～-8.8.These data indicate that the gabbroic xenoliths have characteristics consistent with those of alkaline basalt formed in continental rift,and support extension in the regional tectonic regime during their formation.The crystallization temperatures of the minerals in the xenoliths with gabbro texture and the"matrix"in the xenoliths with "porphyritic"texture were determined in the range of 1049℃ and 910～933℃,respectively,using the amphibole-plagioclase and Al-in-amphibole geothermometers.The pressures determined based on electron microprobe analyses of these phases and the"phenocryst"in the xenoliths with "porphyritic" texture indicate that they were formed at the depths of 26 km,8～10 km,and 13 km.These temperatures and depths represent the conditions of magmatic crystallization corresponding to a deep-level magma chamber and a shallow-level magma chamber,respectively.The depth of the deep-level magma chamber is correlated to the depth of the lower sima crust of the lithosphere,and the depth of the shallow-level magma chamber is at the depth between the Arehean Group- Lower Proterozoic Series of hypometamorphic rocks and the Middle-Upper Proterozoic Series of epimetamorphic rocks.Based on the facts mentioned above,it may be inferred that a continent- continent collision between the Yangtz and North China Cratons in the Triassic followed by a continental extension led to the transformation of the regional tectonic regime from early (T2-T3) compression to late(J3一K1)extension and fault depression,and resulted in a regional magma activity of alkaline basalts.The alkaline basaltic magma was derived from upper mantle,and underplated at the depth of 26km± at about 140Ma.After this,crystallization of the underplated alkaline basaltic magma attributed to the formation of the xenoliths with gabbro texture in the deep-level magma chamber at the depth of 26km±.The relevant Sr and Nd isotope results and petrological and mineralogical data are consistent with assimilation of the underplated alkaline basaltic magma with the lower sima crest in the formation of the derived magma with gabbroic composition in the deep-level magma chamber,and support formation of the magma with dioritic composition in the shallow-level magma chamber at the depth of 8～13 km by assimilation of the derived magma from the deep-level magma chamber with the metamorphic rocks.Crystallization fractionation of the dioritic magma in the shallow-level magma chamber Was responsible for formation of the"phenocrysts"in the xenoliths with "porphyritic" texture.and crystallization of the derived magma intruding from the deep-level magma chamber into the shallow-level magma chamber resulted in formation of the"ma trix"around the earlier "phenocrysts" in the xenoliths with "porphyritic"texture.