Magmatic Evolution And Its Relationship With Metallogenesis In Erenhot-Dongwuqi Area

Erenhot-Dongwuqi area is located to the north of Erenhot-Hegenshan tectonic mélange zone and in the superimpose-convergent part of tectonic domains of Paleoasian Ocean and Paleo-Pacific Ocean. Widespread magmatic rocks and abundant mineral deposits are distributed in the area. It is of great significance to use magmatic rock assemblage as the “scale” to investigate tectonic evolution and as the “probe” to reveal deep process, thereby to reconstruct tectonic evolution process and identify major tectonic settings of ore-genesis.By integrating big data of 1:50000 regional geological surveys which covers almost the whole areas, we constructioned a regional geological map, examined the spatial-temporal distribution and found that magmatic activities in Erenhot-Dongwuqi area mainly concentrate in four periods of time: early Paleozoic(470~430Ma), late Paleozoic(370~260Ma), early Mesozoic(250~210Ma) and late Mesozoic(250~210Ma), in which the most intensive magmatic activities occur in two periods of time of 330-290 Ma and 170-120 Ma.According to the above mentioned four periods of time, taking intrusion episode as basic unit, we investigate the spatial distribution and composition of magmatic rocks, study the geochemical characteristics of typical magmatic rocks of different ages, make an attempt on utilizing big data from 1:50000 regional geological surveys to carry out geochemical comparison in the whole area, and then divide the igneous rock of the area into three tectonic assemblages:(1) the early Paleozoic tectonic assemblage of magmatic rocks(470~430Ma), including middle Ordovician island arc magma assemblage, late Ordovician magma assemblage of transitional environment of island arc- post orogeny, and early Silurian post orogenic magma assemblage;(2) the late Paleozoic- early Mesozoic magmatic tectonic assemblages(>370~210Ma), including the early-middle Devonian island arc(post arc) volcanic rock, the late Devonian paralic volcanic rock assemblage of extensive island arc settings, the early Carboniferous Period continental marginal(inner arc) rock assemblage, the late Carboniferous – early Permian island arc paralic volcanic rock assemblage, the late Carboniferous – early Permian post collision – post orogenic magmatic rock assemblage, the early Permian- middle-late Triassic post orogenic rock assemblage, the late Triassic within-plate magmatic rock assemblage;(3) the late Mesozoic magmatic tectonic assemblage(180~110Ma), including the middle-late Jurassic post collision- post orogenic magmatic rock assemblage and the early Cretaceous post orogenic magmatic rock assemblage. It can be obtained from combination of regional geological researches that magmatic assemblages of the early Paleozoic are related to South-Mongolia Ocean, those of the late Paleozoic – early Mesozoic are closely related to Paleoasian Ocean evolution, the late Mesozoic magmatic tectonic assemblage is related to Paleo-pacific Ocean evolution. Transferring of the latter two ocean evolutions maybe occur in late Triassic.By summary of the spatial-temporal relation between the known ore deposits and magmatic rocks, we find that ore forming process concentrate in three periods of important magmatic evolution: late Carboniferous – early Permian(320~270Ma), the magmatic evolution of transitional period from post collision and post orogeny, which is the essential factor that dominate metallogeneses of molybdenum, copper and other metals; early Mesozoic- Triassic(237Ma±), the post orogenic magmatism after the close of Paleoasian Ocean, which formed multiple mineralization of iron-zinc, lead-zinc-silver etc.; early Cretaceous of late Mesozoic(130Ma±), the period of magmatic evolution in post orogeny related with Paleo-pacific Ocean, accompanying large scale metallogenesis, is the most important period of polymetal metallogenesis.