Petrogenesis Of Late Mesozoic Potassic And Sodic Volcanic Rocks In The Southern Parts Of Yishu Deep Fault Zone And Its Implications For Lithospheric Thinning Beneath The North China Craton

In this paper, representative Late Mesozoic potassic and sodic volcanic rocks from Tangtou and Shenquan in the southern parts of Yishu deep fault zone have been selected for detailed studies on petrology, mineral chemistry, whole-rock elemental and Sr-Nd-Pb isotopic geochemistry, and zircon U-Pb geochronology and Lu-Hf isotopic geochemistry. Based on these studies, this paper has obtained their accurate crystallization ages, and revealed the temporal relationship, and petrological, elemental and isotopic geochemical differences, and thereby elucidated the petrogenesis and the possible factors controlling the chemical variations between the sodic and the potassic volcanic rocks, and further discussed their possible implications for lithospheric thinning beneath the North China Craton. Finally, a possible model coupled the chemical evolution of volcanic rocks and lithospheric thinning process has been proposed.The Tangtou basin in Shandong Province is located in the southern segment of the Yishu fault zone. Late Mesozoic latitic volcanic rocks belonging to alkaline series are widely developed in this basin. Chemically, these volcanic rocks can be further distinguished into a potassic type and a sodic type, of which the former is the predominant type in the basin. The potassic type consists mainly of biotite latitic pyroclastic rocks and subvolcanic rocks, and the sodic type comprises mainly of pyroxene trachyandesitic subvolcanic rocks. LA-ICP-MS zircon U-Pb dating yields an age of (124.0±1.3) Ma for the potassic rocks, and (106.4±4.0) Ma for the sodic rocks, indicating that the sodic rocks were crystallized in a late stage. The potassic and the sodic volcanic rocks show systematically geochemical differences. Compared with the sodic volcanic rocks, the potassic rocks are more enriched in alkalis, with (K2O+Na2O) contents of9.11%～11.37%and of7.96%～8.93%, respectively. All the volcanic rocks are enriched in LREE and LILE, but the potassic rocks have higher REE (especially LREE) contents relative to the sodic rocks, with EREE values of360.1～417.Oppm and (La/Yb)N ratios of45.72-64.66for the potassic rocks and of173.8～291.Oppm and33.80～40.52for the sodic ones. The potassic rocks also show higher LILE (e. g., Cs, Rb, Ba, Th, U, Pb, etc.) and HFSE (e. g., Zr, Hf) concentrations, but lower Sr, Ti abundances. All the samples have enriched Sr and Nd isotopic compositions (specially0.7107～0.7119and～15.48～16.96for the potassic rocks, and0.7097～0.7100and～10.03～11.83for the sodic rocks), similar to that of Mesozoic mafic rocks in the south of Luxi, indicating that they were originated from an enriched mantle source. This is further supported by the negative εHf(t) values of zircons (-13.3～-19.8for potassic rocks and-12.2～-16.6for sodic rocks, respectively) from the two types of rocks. Integrated studies suggested that the source enrichment for the volcanic rocks in Tangtou basin were mainly induced by delamination of the lower crust of the North China Craton, but it was also affected by the previously subducted Yangtze crustal materials.The Shenquan sodic volcanic rocks, outcropped in the east of Lizhuang village in Tancheng County, Shandong Province, are tectonically located in the southern parts of the Yishu deep fault zone. The volcanic rocks comprise a shield volcano, and lithologically consist mainly of trachyandensite. Zircon LA-ICP-MS U-Pb dating yields an age of (96.5±1.4) Ma, which is younger than the ages of the widespread potassic rocks within the region, indicating a younging evolution trend from potassic to sodic for the Mesozoic volcanic rocks along the Yishu deep fault zone and its adjacent region. Chemically, the Shenquan volcanic rocks have high alkali contents (Na2O+K2O=10.34%～11.95%) and obviously enriched in sodium (Na2O/K2O=1.76～3.92), and can be ascribed to typical sodic volcanic rocks by using various discrimination schemes. Geochemically, the rocks contain relatively high concentrations of rare earth elements (EE=411.3～456.3ppm), and are enriched in LREE (LREE/HREE=19.53～21.10,(La/Yb)N=29.69～33.26) and LILE (e. g., Rb, Ba, Th), and depleted in HFSE (e. g., Ti, Nb, Ta). They show right-inclined chondrite-normalized REE distribution patterns and display somewhat negative Pb anomalies in the primitive mantle-normalized trace element spidergrams. These rocks have enriched Sr-Nd isotopic compositions (Isr=0.7068～0.7077,d(0=-16.66～-16.82). The Pb isotopic ratios are (206Pb/204Pb)1=16.407-17.100,(207Pb/204Pb)i=15.253～15.387,(208Pb/204Pb)i=36.515～37.266, with all the samples plotted above the NHRL and shown a good linear correlation between the Pb isotopic ratios. Based on a synthesis of the geology and geochemistry, we suggest that Shenquan sodic rocks were derived from an enriched mantle, but their mantle source might have modified by multiple events, including source contamination of continental crustal materials from deep subduction of the Yangtze Craton during Triassic and subsequently lithospheric delamination of the North China Craton. During Late Cretaceous, the continued extension of Tanlu Fault induced the upwelling of asthenospheric melts which further modified the previously enriched mantle. Decompression partial melting of these multiple modified mantle source and further differentiation of the magmas finally generated the Shenquan sodic volcanic rocks.The chemical compositions of volcanic rocks in Tangtou basin evolved from potassic to sodic. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the potassic and the sodic volcanic rocks in this basin are different, with the sodic rocks show slightly low Isr ratios, and relatively high whole-rockεNd(t) and zircon εHf(t) values, indicating that their magma might be generated from different sources. Binary mixing simulation demonstrates that the magma source of the sodic rocks contain higher proportion of depleted asthenosphere mantle components. Based on a synthesis of geochronology, elemental and isotopic geochemistry and tectonics, we propose that the variation of the source components between the potassic and the sodic rocks in Tangtou basin was most likely generated via the following processes, i.e., the regional extension intensity increased with time led the Yishu deep fault cut in a deeper level, then induced the upwelling of asthenosphere and thus resulted the source region of the sodic volcanic rocks contained a higher proportion of depleted asthenosphere mantle components. The Mesozoic volcanic rocks in other districts along the Yishu deep fault zone and its adjacent areas also show the same chemical evolution trend from potassic to sodic with time. This chemical evolution trend should be mainly controlled by heterogeneous of their mantle sources, which was most likely induced by the upwelling of asthenosphere under a sustained tensional setting. The chemical compositions of volcanic rocks evolved from potassic to sodic directly responses to the replacement of lithosphere mantle by the upwelling asthenosphere mantle, which ultimately led the lithosphere thinning of the North China Craton.