Studies On Geochemical And Zircon U-Pb Dating Of Volcanics From Erlangping Group Of The North Qinling, Eastern Qinling Mountains

Erlangping group is an important part of the North Qinling orogenic belt, and record a lot of information of formation and evolution of the North Qining orogeny. Many studies have been contributed to its formation age and tectonic setting, but there still are some disputes. Volcanic rocks of the Erlangping group are well distributed at the western Henan province. Based on the detailed investigation of field occurrence, this thesis focus its studies on the tectonic setting and the formation age of the Erlangping Group volcanic rocks, using integrated studies of petrology, geochemistry and zircon U-Pb geochronology for the volcanic rocks outcrop on the western Henan Erlangping regions in Junmahe. Wantan and Tuanhe-Tuanvuan section.The results will provide important constraints on discussing the structural relationship between the Erlangping Group and the Qinling Group and further discussing the tectonic framework and evolution of the North Qinling in Paleozoic period. The main progresses of this thesis are as follows:1. Detailed geological and petrography investigations indicate that the Erlangping Group volcanics show spilite-keratophyre construction with spilite as the mainly. The spilite is greyish-green, display massive structure and locally pillow structures with palimpsest interweave texture and intergranular texture. It is mainly composed of chlorite, albite, epidote and minor quartz, titanium magnetite; keratophyre-quartz keratophyre is gray-green to gray, occur as interlayer or lenses with clear boundary within the spilite, and display blastoporphyritic or blastobedding structure. The phenocrysts are mainly albite and quartz, and the matrixes are mainly composed of fine-grained quartz, albite and potassium feldspar.2. Geochemical studies show that spilite-keratophyre from Erlangping Group is formed in back-arc basin environment. The spilite have SiO2contents vary from45.92%to52.97%, Na2O from2.24%to4.62%, and TiO2from0.54%to1.16%which fall in the area of island arc basalts and mid-ocean ridge basalt. The spilites from Junmahe and Tuanhe section exhibit flat LREE pattern which is similar to the REE pattern of MORB; while the spilite from Wantan section displays LREE lightly enrichment pattern, showing the characteristic of island arc volcanic rocks. The rocks from the three sections are all enriched in LILE, such as Sr, Ba, Th, and slightly depleted in HFSE such as Nb and Ta, exhibiting the feature of both ocean ridge basalt and island arc basalt. And all samples fall in the mixing zone of the volcanic arcs and N-MROB on the trace element tectonic discrimination diagrams. Keratophyre samples have SiO2content range from62.28%to72.22%, Na2O range from3.81%to4.80%with average of4.16%, K2O from0.11%to0.31%with average of0.18%. Quartz keratophyre samples have SiO2content vary from73.60%to77.64%. Na2O range from4.43%to5.09%with average of4.83%, K2O from0.11%to0.33%with average of0.29%.These two rocks all show the characteristics of high Si and Na, low Ti and K, and the Na2O/K2O ratio is of12.77-42. They also have higher total REE content and display right lean REE patterns with LREE enrichment and HREE depletion, and a weak negative Eu anomaly. They show similar features ocean ridge granitein ORG normalized trace elements spider diagram, whereas, the depletion of Nb and Ta indicates their subduction-related origin. Therefore, all these geochemical characteristics of spilite, keratophyre and quartz keratophyre suggest that the Erlangping Group should be formed at the geological setting of back-arc basin resulted from oceanic crust subduction.3. Cathode luminescent images and in situ trace element analyses show that, zircons from the Erlangping group spilite and keratophyre-quartz keratophyre display oscillatory zoning internal structures and obvious HREE enrichment REE pattern, the typical magmatic zircon. LA-ICP-MS in situ U-Pb dating obtains the formation ages of (463±1.8) Ma,(475±1.5) Ma and (473±1.3) Ma for spilite from the Junmahe, Wantan and Tuanhe section, respectively, and (468±1.1) Ma for quartz keratophyre from Wantan section. These ages are well consistent with each other within errors, indicating that the volcanics of the Erlangping Group in the North Qinling Orogen were formed during463-475Ma.4. Combined the results of petrography and geochemistry with geochronology, it is suggest that the Erlangping Group volcanic rocks are a spilite-keratophyre construction which formed during463-475Ma in a back-arc basin setting which related to the oceanic crust subduction.