Mantle plume related Paleozoic riftogenic alkaline magmatism of Mongolia: isotope and geocemical evidence.
Abstract
Paleozoic alkaline magmatic activity in Mongolia associates with series of W-E trending rifts, ranged from south to north. The rifts system was evolved northward at three stages beginning from later Carboniferous-earlier Permian to later Permian. The isotopic-geochemical composition of the rocks from five volcano-plutonic bimodal complexes was studied. These complexes belong to the Gobi-Tien Shan Zone (GTZ) 310-290 Ma and the Gobi-Altai Zone (GAZ) 270 Ma of riftogenic alkaline magmatism. The studied volcano-plutonic complexes are formed by mafic rocks alkaline basalts and felsic rocks alkaline riolites (comendite and pantelerite) with subordinate rocks of intermedial composition (trachidacites, trachiandesites) and associated plutons of alkaline granites. Basaltic rocks of GTZ are characterized by positive eNd(T) values (+4.2 +7.0), 87Sr/86Sr(T) value ranged from 0.7036 to 0.7049, enrichment in Th, LREE, Sr, P, Zr, Hf, Ti and pronounced depletion in Nb and Ta. GTZ felsic rocks have general enrichment in most incompatible trace element (K, Rb, Th, Zr, Hf and REE) and depletion in Nb, Ta, Sr, P, Eu and Ti. The range of eNd(T) of alkaline riolites demonstrates general shift to more positive values (+5.8 +7.3) comparatively to that of basalts. Generally rocks of GAZ and GTZ volcano-plutonic complexes have similar geochemical features. However the GAZ alkaline riolites have more differentiated character (higher silica content, more strong depletion in Ti, Sr and Eu, enrichment in Rb and REE). Furthermore eNd(T) and 87Sr/86Sr(T) values of GAZ basalts demonstrate more wide scatter from +0.2 to +8.1 and from 0.7037 to 0.7054 respectively. Geochemical trends in the trace elements (REE, Y, Th, Nb, Ta) and in Sr-Nd isotope composition suggest that basaltic rocks were derived from OIB-like mantle sourse and underwent contamination during of fraction crystallization by material of subduced oceanic crust (Neoproterozoic ?). Felsic rocks were generated from OIB-like basaltic melt trough fraction crystallization without of appreciable assimilation of oceanic crust material. Evolution in chemical and Nd and Sr isotope composition of mafic volcanic rocks from GTZ to GAZ reflects change in tectonic settings of riftogenic magmatism caused by drift of litospheric slab under the mantle diapir (mantle plume), that triggered sequential rifts formation during the Upper Paleozoic in Mongolia.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA......686Y