Granitic batholith emplacement mechanism in a transtensional setting: petromagnetic evidence from the Southern Urals

Introduction. The Nepluyevka pluton is the Early Carboniferous polyphase batholith situated in the East Ural zone. The batholith is subdivided into 4 phases ranging from basic to felsic in composition. The pluton formed during the Early Sudetian orogenic phase of the East Ural zone, which was characterized by a complex alternation of pure and sub-simple shear kinematic settings over its’ duration. Evidently, this alternation was connected with changes in the kinematics of the subduction zones preceding the Late Visean collision of Laurussia and Kazakhstania. These transformations had defined the characteristic features of the tectono-magmatic evolution of the southern part of the East Ural zone. Thus, their investigation is crucial for improving our understanding of the geological history of the Southern Urals.

Methods and materials. We have investigated anisotropy of magnetic susceptibility (AMS) and magnetic mineralogy of the rocks of the Nepluyevka batholith to gain insights into the circumstances of its’ formation and its’ deformation history. Totally 186 oriented specimens from 16 sites spread over all the phases of the pluton were collected. MFK-1A kappa-bridge was used to measure MS and AMS, temperature dependencies of induced magnetization were studied with Curie balance, magnetic hysteresis loops were obtained on J_meter coercivity spectrometer.

Results. The specimen appeared to contain PSD high-Ti magnetite (magmatic), MD low-Ti magnetite (hydrothermal), as well as the minerals of goethite and maghemite-hematite series.

The AMS data tells the history of the formation and structural evolution of the batholith. Gabbro (1st phase) and granodiorites (2nd phase) in the center of the pluton are characterized by prolate magnetic fabrics. Lineation there is steep to sub-vertical and marks the flow direction near the feeder shared by both phases. Granodiorites (2nd phase) in the north and adamellites (3rd phase) in the north and the south of the pluton are characterized by predominantly oblate, flat-dipping fabrics, corresponding to lateral spreading of the melt. The magnetic fabrics of the adamellites (3rd phase) near the pluton’s southern boundary are oblate and dip steeply in the SW direction, marking the melt flowing parallel to the contact. The magnetic fabrics of the adamellites in the NE part of the batholith are similarly oblate and subparallel to the contact.

Discussion. We propose the model of “magmatic duplex” for the formation of the pluton. The upper-crust transtensional structure associated with a sinistral strike-slip fault was draining the lower-crust magma chamber. Due to the fractionation and assimilation of the chamber’s wall material, it was supplying increasingly felsic melt. Formation of the first two phases was controlled mainly by the central feeder. The 3rd phase adamellites intruded two weakened contact zones of the pluton as the transtensional structure continued to grow sub-longitudinally. The pluton has experienced secondary heating and some metasomatic alteration, but no significant deformations occurred.

Financial support. The research has been funded by RFBR and CNF as a part of the research project № 19-55-26009 with the use of materials of the "Geoportal" Center of the Lomonosov Moscow State University.