Emplacement Mechanism Of Longsheng Granitic Pluton,Northern Guangxi

Longsheng granite pluton is nearly SN-oriented,spindle-like,and concentric,and crops out in the core of Longsheng anticlinorium of northern Guangxi.The pluton consists of the inner Lazheng granodiorite and the outer Tongpen monzogranite.The Lazheng granodiorite belongs to calc-alkaline series;two samples collected from the granodiorite yielded zircon U-Pb ages of 436.0 ± 13.0 Ma and 437.0 ± 6.0 Ma,indicating that it was emplaced in Landovidian Epoch of Silurian Period.The Tongpen monzogranite mainly belongs to calc-alkaline series as well;two samples collected from the monzogranite gave zircon ages of 424.4±5.0 Ma and 425.1±5.2 Ma,suggesting its emplacement in the Ludlow Epoch of Silurian Period.The geochemical compositions of both plutons demonstrate that the magmas were sourced from the crust as a result of partial melting of meta–mudstone in a post-collision tectonic environment dominated by extension;and the pluton is a typical S-type granite.Magnetic fabric measurement within both plutons shows magnetic susceptibility anisotropy(P)from 1.02 to 1.09,indicating weak deformation.The E value of the susceptibility ellipsoids ranges between 0.951 and 0.998,indicating a mainly elongation strain.The magnetic foliation near the contact of the pluton is mostly parallel to the contact,but it is more disordered in the interior of the pluton,suggesting that the magma may initially ascend along a crack,and then fill the inner space disorderly.The preferred orientation of the magnetic lineation indicates that the magma moved roughly from south to north.Generally,the Lazheng granodiorite magma and the Tongpen monzogranite magma were emplaced along a longitudinal tension fissure opened along the hinge zone of the SN-trending Longsheng anticlinorium in a dike propagation mechanism.After the Lazheng granodiorite was emplaced within the fissure,the Tongpen monzogranite was emplaced along the outer contact of the already-emplaced Lazheng granodiorite and due to magma floating at higher crustal level,the magmas of the Tongpen monzogranite were also emplaced above the Lazheng granodiorite,forming a concentric pattern for the entire pluton,forming an outward-incremental growth pattern.