Field investigations and laboratory integrated research as indicated that ophiolite mélange in the Nan-Uttaradit zone, northern Thailand, consists of fragments of tectonites such as metamorphic peridotite (extremely silicified serpentinite), cumulates (pyroxenolite, gabbro, and gabbro-diorite), ocean-ridge basalt, oceanic-island ba-salt and radiolarian silicalite, and it was formed during D3-P. The rock series, rock types and petrogeochemical characteristics of metamorphic tholeiites in the Nan area of the Nan-Uttaradit zone are similar to those of ocean-ridge basalts (C1) in China's Ailaoshan zone. As for the Hawaiites in the Nan area of the Nan-Uttaradit zone, their major elements, REEs and trace elements are similar to those of oceanic-island basalts in China's Jinshanjiang zone (P11). In the Uttaradit area of this zone the metamorphic alkaline basalts show transitional petrogeochemical characteristics between ocean-ridge basalts and oceanic-island basalts, which were still formed in oceanic-island environments. The above-described basalts are all oceanic volcanic rocks and they are the most important part of the Paleo-Tethys oceanic crust in the Nan-Uttaradit zone.
On the basis of the petrographic characteristics, rock assemblages, petrochemistry, REEs, trace elements and geotectonic settings, the authors described the characteristics of continental marginal arc-volcanic rocks in the Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of the ocean-ridge/oceanic island basalts in the Chiang Mai belt. The volcanic rock assemblage is basaltic andesite-andesite-rhyolite. The volcanic series is dominated by the calc-alkaline series, with the tholeitic series coming next. The chemical composition of the volcanic rocks is characterized by high Al2O3; the REE distribution patterns are of the LREE-enrichment rightward incline type; the large cation elements are highly enriched, and the volcanic rocks are generally enriched in U and Th and depleted in Ti, Cr and P. The petrochemical plot falls within the field of island-arc volcanic rocks, in consistency with the projected points of continental marginal arc-volcanic rocks in the Lancangjiang belt. These continental mar-ginal arc-volcanic rocks, together with ocean-ridge/oceanic island-type volcanic rocks in the Chiang Mai belt, con-stitute the ocean-ridge volcanic rock-arc magmatic rock belts distributed in pairs, indicative of eastward subduction of the oceanic crust in the Chiang Mai belt. This result is of great importance in exploring the evolution of the paleo-Tethys in the Chiang Mai belt.
The oceanic island volcanic rocks in the Chiang Mai zone, northern Thailand, are usually covered by Lower Carboniferous and Upper Permian shallow-water carbonate rocks, with the Hawaii rocks and potash trachybasalt being the main rock types. The alkaline series is dominant with sub-alkaline series occurring in few cases. The geochemical characteristics are described as follows: the major chemical compositions are characterized by high TiO2, high P2O5 and medium K2O; the rare-earth elements are characterized by right-inclined strong LREE-enrichment patterns; the trace element patterns are of the upward-bulging K-Ti enrichment type; multi-component plots falling within the fields of oceanic island basalts and alkali basalts, belonging to the oceanic island-type volcanic rocks, which are similar to the equivalents in Deqin and Gengma (the Changning-Menglian zone) of Yunnan Province, China.
The widespread Permian carbonate strata outcropped in northwestern Thailand are considered as the evidence for the Late Paleozoic shallow Tethys. Our investigation, however, shows that basalt can be discovered usually under the Permian carbonate sequence
Late Permian-Early Triassic(P2-T1) volcanic rocks distributed on the eastern side of ocean-ridge and oceanic-island basalts in the Nan-Uttaradit zone were analyzed from aspects of petrographic characteristics,rock assemblage,REE,trace elements,geotectonic setting,etc.,indicating that those volcanic rocks possess the characteristic features of island-arc volcanic rocks.The volcanic rock assemblage is basalt-basaltic andesite-andesite.The volcanic rocks are sub-alkaline,dominated by calc-alkaline series,with tholeiite series coming next.The chemical composition of the volcanic rocks is characterized by low TiO2 and K2O and high Al2O3 and Na2O.Their REE patterns are of the flat,weak LREE-enrichment right-inclined type.The trace elements are characterized by the enrichment of large cation elements such as K,Rb and Ba,common enrichment of U and Th,and depletion of Nb,Ta,Zr and Hf.The petrochemical plot falls within the field of volcanic rocks,in consistency with the plot of island-arc volcanic rocks in the Jinsha River zone of China.This island-arc volcanic zone,together with the ocean-ridge/oceanic island type volcanic rocks in the Nan-Uttaradit zone,constitutes the ocean-ridge volcanic rock-island-arc magmatic rock zones which are distributed in pairs,indicating that the oceanic crust of the Nan-Uttaradit zone once was of eastward subduction.This work is of great significance in exploring the evolution of paleo-Tethys in the Nan-Uttaradit zone.
