THE FERRAR LARGE IGNEOUS PROVINCE, ANTARCTICA: FIELD AND GEOCHEMICAL EVIDENCE BEARING ON MODES OF EMPLACEMENT

The Ferrar Large Igneous Province (LIP), a linear belt (>4000 km) across Antarctica and into SE Australasia, presents a case study for evaluating the dispersal of LIP magmas. It comprises numerous sills, a layered mafic intrusion, remnants of lava fields, and minor pyroclastic rocks. Geochronology demonstrates a restricted duration of emplacement (<0.4 m.y.) at ∼182.7 Ma. Sills mainly occur in flat-lying Beacon strata. Dike swarms have been inferred locally by geophysics.

Ferrar rocks show great geochemical coherence and consist of two chemical types. One, designated the MFCT, forms most of the province and the majority of analyzed rocks. It has a range of compositions (MgO ~9-3%; Sr_i ~0.709-0.712), which are related by fractional crystallization and ~5% crustal assimilation. The remaining 1% (by volume), designated the SPCT, has a distinct, evolved, and restricted composition (MgO ~2.3%; Sr_i ~0.7095), which lies off MFCT chemical trends. The SPCT comprises the youngest lavas in the Transantarctic Mtns and minor sills in the Theron Mtns. The chemical consistency suggests a single melt source most likely in the proto-Weddell Sea region, rather than a linear set of sources with variable melting conditions and magmas that would have interacted with crustal provinces of varying ages and isotopic characteristics.

Lateral transport models for Ferrar magma dispersal have invoked both dike and sill transport. Laterally extensive sills are well documented and lateral transport for 10s if not 100s of km is permissive. However, we consider province-wide transport (1000s of km) via shallow level sills in Beacon strata improbable because dikes and sills intrude basement granite, and magmas would have to overcome paleogeographic highs, before descending through 100s of meters of section.. The preferred scheme is for long distance magma transport at the crust-mantle boundary or in the lower crust as dikes, with widely spaced centers of enhanced vertical transport documented by thick aggregate sill accumulations and extant extrusive rocks . Magma density (proxied by MgO%) or magma migration upward though a dike and sill network in a neutral stress field have been invoked to control vertical transport at shallow levels in the plumbing system. Regional geochemical relations suggest that neither mechanism operates province wide.