The Western Pansky Tundra layered intrusion, kola peninsula: Differentiation mechanism and solidification sequence

Hypotheses of the genesis of the Western Pansky Tundra layered intrusion di ffer in the interpretation of some key problems in the petrology of the plu ton. These include: (1) whether the pluton is an individual geologic body o r a part of the larger Pansky and Fedorovo-Pansky massifs; (2) whether it i s a mono- or polyphase pluton, (3) what is the crystallization trend of the parental melt; (4) which processes were responsible for the development of the two critical zones of the intrusion, the Lower and the Upper Layered U nit (LLU and ULU, respectively); and (5) whether the magnetite gabbro is a foreign or syngenetic rock. Based on the analysis of preexisting data, a mo del was proposed for the evolution of the Western Pansky Tundra intrusion a s an individual pluton. The massif is proved to consist of two intrusive ph ases, the first of which comprises the lower, gabbronorite portion of the m assif, from its bottom to the Upper Layered Unit, and the other extends fro m the Upper Layered Unit to the overlying gabbronorite. The parental melt o f the first intrusive phase corresponded to unsaturated tholeiite (an(L) = 65%, f(L) = 23%). The crystallization of this phase proceeded from bottom t o top and, later, when the chamber was divided into a series of smaller res ervoirs (subchambers), from their walls to centers. The genesis of the Lowe r Layered Unit is related to a temporary opening of the chamber and its rep lenishment with batches of melt whose composition was close to that of the parental magma. The differentiation trend of the parental melt of the first intrusive phase was characterized by the following crystallization success ion of minerals: Pl + Opn (norite) --> Pl + Opx + Cpx (gabbronorite) --> Pl + Cpx + Opx + Pig (gabbronorite with inverted pigeonite) --> Pl + Opx + Op x + Pig + Mgt (magnetite gabbro with inverted pigeonite). The second intrus ive phase was produced by a later voluminous melt portion, whose compositio n was less evolved than that of phase I (an(L) - 75%, f(L) = 19%). The crys tallization of the original melt of phase II started with the cotectic asse mblage Pl + Opx + Cpx (gabbronorite). Later, brief disturbances of the diff erentiation trend occurred at the level of olivine-bearing beds within the Upper Layered Unit because of the injection of melt batches of olivine thol eiite composition. The crystallization succession in these beds was general ly as follows: Pl (+/- Ol) (olivine leucogabbro) --> Pl + Ol (troctolite) - -> Pl + Ol + Opx(olivine norite) --> Pl + Opx (norite) --> Pl + Opx + CPx ( gabbronorite).