Bashkirian Sr-Nd age of the terminal stage of collisional magmatism in the Western Magnitogorsk zone of the South Urals

Abstract

Magmatism in the Western Magnitogorsk zone ofthe Southern Urals took place on the accretionarybasement (island arc basement partly thrust over thecontinental margin) and is represented by mafic (lesscommonly, intermediate, ultramafic, and acid) rocksthat constitute intrusions of variable morphology: sillshaped sheets, laccoliths, lopoliths, chonoliths,stocks, dikes [1]. Based on their geological positionand petrological properties, these intrusions are unitedinto several complexes, which were formed during several stages. Each magmatic stage was characterized bylocal tectonic settings that existed from the Tournaisian to the Bashkirian ages [2, 3]. In our opinion, theoldest, Carboniferous rocks are related to the late stageof the collision between the Magnitogorsk island arcand passive margin of the East European continent,which is termed in [4] as a “soft collision,” laterreplaced in the Middle Carboniferous [5] by a “hardcollision,” i.e., a collision between the Kazakhstanand East European blocks. Until recently, the age estimates of magmatism in the Western Magnitogorskzone were based on geological and petrological features: stratified complexes with migmatites, intersection of intrusive bodies and dikes, and a compositionalanalogy with Tournaisian–Visean basalts of the neighboring Magnitogorsk–Bogdanovsk graben [6]. TheKhudolaz complex of differentiated and layered bodies with sulfide Cu–Ni mineralization recentlybecame the sole exception: baddelyite and zircongrains extracted from two autonomous gabbroid bodies yielded similar isotopic dates of 324.78 ± 0.46 and328.90 ± 0.78 Ma, respectively [7]. The terminal stageof magmatism in the Western Magnitogorsk zone isrepresented by a dike belt (dike complex), whichcrosses all the complexes of the Khudolaz syncline andstrikes at NE 10°–13° (Fig. 1).The available data on the Khudolaz complex implyits formation in the environment of early stabilizationof the island arc basement [6], i.e., in the subplatformal regime. This stage likely characterizes the tectonicpause between the two abovementioned collisiontypes. For a long time, it remained unclear whether ornot the dike complex is synplutonic, i.e., representinga terminal stage of magmatism in the Nibearing Khudolaz complex as was assumed by E.K. Buchkovskii etal. in their stored reports of 1971 and 1974, or if it ispostmagmatic, i.e., reflecting an autonomous latemagmatic stage as proposed in [1]. At the same time,inasmuch as no data on the upper geological limit ofthe dike complex were available at that time, we conceded both the Late Carboniferous and Early Permianage for the latter. The recently obtained date of 321 ±15 Ma supports our previous conclusion that the dikecomplex represents an autonomous stage of magmatism and precedes the commencement of hard collision in the Western Magnitogorsk zone. This was thelast pulse of magmatism in this structure.The thickness of dikes in the complex under consideration is sustained along the strike averaging 0.5–1.2 m. In areas where dikes cross the sedimentarycomplexes, the length of their outcrops is maximum(>1 km). The dikes are frequently bifurcated to joinagain or extend parallel to each other over a significantdistance.The mineralogical composition and texture ofrocks in dikes allow the following types to be defined:hornblende dolerites, dolerite porphyrites, gabbrodiorites, lamprophyres (spessartite, malchite), andgabbro dolerites. The rocks are variably altered:pyroxenes are partly or entirely replaced by secondaryhornblende (uralite, actinolite, tremolite); plagioclaseis frequently sossuritized; basaltic hornblende is commonly replaced by uralites and ore mineral. Fissures inrocks are filled with serpentine, chlorite, and epidote.Some relatively fresh varieties exhibit both unalteredplagioclase and clinopyroxene.