Three-dimensional model of an ultramafic feeder system to the Nikolai Greenstone mafic large igneous province, central Alaska Range

Abstract

[1] The Amphitheater Mountains and southern central Alaska Range expose a thick sequence of Triassic Nikolai basalts that is underlain by several mafic-ultramafic complexes, the largest and best exposed being the Fish Lake and Tangle (FL-T) mafic-ultramafic sills that flank the Amphitheater Mountains synform. Three-dimensional (3-D) modeling of gravity and magnetic data reveals details of the structure of the Amphitheater Mountains, such as the orientation and thickness of Nikolai basalts, and the geometry of the FL-T intrusions. The 3-D model (50 × 70 km) includes the full geographic extent of the FL-T complexes and consists of 11 layers. Layer surfaces and properties (density and magnetic susceptibility) were modified by forward and inverse methods to reduce differences between the observed and calculated gravity and magnetic grids. The model suggests that the outcropping FL-T sills are apparently connected and traceable at depth and reveals variations in thickness, shape, and orientation of the ultramafic bodies that may identify paths of magma flow. The model shows that a significant volume (2000 km 3 ) of ultramafic material occurs in the subsurface, gradually thickening and plunging westward to depths exceeding 4 km. This deep ultramafic material is interpreted as the top of a keel or root system that supplied magma to the Nikolai lavas and controlled emplacement of related magmatic intrusions. The presence of this deep, keel-like structure, and asymmetry of the synform, supports a sag basin model for development of the Amphitheater Mountains structure and reveals that the feeders to the Nikolai are much more extensive than previously known.

Multiple negative carbon-isotope excursions during the Carnian Pluvial Episode (Late Triassic)

“…Given their close ages, these repeated disruptions in the carbon cycle could have been related to the emplacement of large masses of volcanic rocks from an igneous province. The Wrangellia LIP, also called the Nikolai LIP (Schmidt and Rogers, 2007;Greene et al, 2010;Glen et al, 2011), is indicated as the most important contributor, possibly strengthened by other basaltic and acidic volcanic sources, namely the South Taimyr igneous complex in Siberia plus the Kara Dere-Sayrun unit of the middle Antalya nappes and the Huglu-Pindos volcanics in Turkey (Dal Corso et al, 2012;Mueller et al, 2016a;Sun et al, 2016;Miller et al, 2017).…”

Gravity and magnetic modelling of layered mafic–ultramafic intrusions in large igneous province plume centre regions: case studies from the 1.27 Ga Mackenzie, 1.38 Ga Kunene–Kibaran, 0.06 Ga Deccan, and 0.13–0.08 Ga High Arctic events

“…Intrusions of this scale (several 10's to several 100's of kilometers in diameter) compare in size to some of the largest mafic-ultramafic intrusions globally, some of which are known Ni-Cu-PGE producers. Some examples include the 50 km wide, 17-19 km thick Freetown igneous complex in Sierra Leone (Mgbatogu et al 1987), the > 30 km wide and 1.5 km thick Fish-Lake and Tangle mafic-ultramafic sills of the Nikolai Greenstone portion of the Wrangellia LIP (Glen et al 2011), and the 500 km wide, 7-9 km thick Rustenburg layered suite of the Bushveld intrusion (e.g. Cawthorn 1996).…”

Geophysical Identification and Characterization of Mafic-Ultramafic Intrusions in Plume Centre Regions

“…For instance, these intrusions compare in size to some of the largest mafic-ultramafic intrusions around the globe, some of which are known Ni-Cu-PGE producers. Some examples include the 50 km wide, 17-19 km thick Freetown igneous complex in Sierra Leone (Mgbatogu et al, 1987), the > 30 km wide and 1.5 km thick Fish-Lake and Tangle mafic-ultramafic sills of the Nikolai Greenstone mafic LIP (Glen et al, 2011), and the 500 km wide, 7-9 km thick Rustenburg layered suite of the Bushveld intrusion (e.g. Cawthorn 1996).…”