Large fields of spodumene pegmatites in the settings of rifting and postcollisional shear–pull-apart dislocations of continental lithosphere

Abstract

The authors analyze the geodynamic settings of large fields of spodumene pegmatites hosting Li and complex (Li, Cs, Ta, Be, and Sn) deposits of rare metals within the Central Asian Fold Belt. Most of the studied fields show a considerable time gap (from few tens of Myr to hundreds of Myr) between the spodumene pegmatites and the associated granites, which are usually considered parental. This evidence necessitates recognition of an independent pegmatite stage in the magmatic history of some pegmatite-bearing structures in Central Asia. The Precambrian–Late Mesozoic interval is marked by a close relationship between the large fields of spodumene pegmatites and extension settings of continental lithosphere. They occur either as (1) zones of long-lived deep faults bordering on trough (rift) structures experiencing the tectonic-magmatic activity or as (2) postcollisional zones of shearing and pull-apart dislocations. Thus, large fields of spodumene pegmatites might serve as indicators of continental-lithosphere extension. Important factors favoring the formation of rare-metal pegmatites both in collision zones and continental-rift settings are the presence of thick mature crust dissected by long-lived, deeply penetrating (down to the upper mantle) fault zones. They ease the effect of deep sources of energy and substance on crustal chambers of granite and pegmatite formation.

New data on the substantial composition of Kalba rare metal deposits

“…The geological-genetic model of pegmatite formation reflects the gradual development of mineral complexes from the graphite and oligoclase-microcline (barren) to spodumene-containing one (non-ferrous) with rich complex ores (Ta, Nb, Be, Li, Cs, Sn). Mineralogical factors for the estimation of rare metal pegmatites are determined by the intensity degree of the metasomatic processes occurrences (microclinization, albitization, greysening, spodumenization, silicification) in them, the indicators of which can be typomorphic minerals (cleavelandite, muscovite, lepidolite, spodumene, colour tourmalines, pollucite, and others) similar for industrial pegmatite deposits of foreign countries (Koktogay, Bernik Lake, King Mountain, and others) [4,5,8,9]. The leading role in ore formation is attached to albitization process, which is the initiator of all subsequent metasomatic transformations of pegmatite veins.…”

Tools and Workflows for Grassroots Li–Cs–Ta (LCT) Pegmatite Exploration

“…However, pegmatites with a missing apparent parental granite are common [3], and it is suspected that the source granite occurs at depth [30]. On the other hand, studies published in the 2010s on the anatectic origin of granitic pegmatites in Europe and North America demonstrate that there are "thousands of pegmatites without parental granites" [13], i.e., pegmatite fields can be unrelated to a source granite and instead form by partial melting and anatexis of crustal material [4,13,14,[35][36][37][38][39], or energy and melt circulation along deep lithospheric fault zones [40]. In the current literature, there are numerous descriptions of pegmatite sub-classifications, but to date there is no universally accepted model explaining the diverse features and genesis of granitic pegmatites [39].…”

A Review of Early Permian (300–270 Ma) Magmatism in Eastern Kazakhstan and Implications for Plate Tectonics and Plume Interplay

“…magmas could not penetrate through thick viscous migmatite-granite lenses ('density filter') [Huppert, Sparks, 1988]. This mechanism includes interaction of the juvenile mantle fluids with the crust or with granitic magma in the chambers, as well as the inputs of some elements responsible for rare-metal mineralization in granites [Abramov, 2004;Annikova et al, 2006;Zagorsky et al, 2014;Sokolova et al, 2016]. Thus, we revealed two main types of mantle-crust interaction: (1) direct interaction of mantle magmas with crustal material and ana- tectic melts that produced large gabbro-granite intrusions, volcanic structures, and numerous small gabbropicrite intrusions in central part of studied region; and (2) the effects of mantle heat and fluids on the crust.…”