麻豆传媒

The ca. 2.67-2.70 Ga Point Lake, Northern Beaulieu River and Central Beaulieu River volcanic belts in the Slave Province, Northwest Territories, Canada, represent portions of a continental arc to back-arc sequence that evolved over approximately 30 m.y. Inferred west-dipping subduction of an oceanic slab beneath sialic crust resulted in the development of a continental arc at least 250 km long. Physical volcanology and geochemistry of the volcanic rocks indicate that the Peltier Formation of the Point Lake belt formed in a back-arc basin overlying continental crust, the Northern Beaulieu River belt represents the transition zone between the arc and back-arc, and the Central Beaulieu Rive belt developed in the arc.

The Peltier Formation overlies the 3.22 Ga Augustus Granite, is interstratified with the >2.65 Ga turbiditic Contwoyto Formation, and is associated with 2.68 Ga felsic-intermediate volcaniclastic rocks of the Samandr茅 and Beauparlant Formations. The mafic-dominated volcanic sequence is consistent with the proximal to distal portions of seamounts in deep to moderate water depths. The proximal facies include a non-vesicular pillowed sequence cut by numerous mafic intrusions, typical of the central, deep water portion of seamounts. The medial to distal, moderate depth facies are represented by pillow breccia, hyaloclastite (peperite), and pillowed and massive flows with 5-27% vesicularity. Interstratified shale units and peperite indicate contemporaneous sedimentation and volcanism. Mafic rocks in the Peltier Formation range from NMORB through tholeiitic LREE-enriched EMORB with Nb and Ti depletions, to strongly LREE-enriched calc-alkaline compositions with Nb and Ti depletions. 脦Nd values are from -2.94 to +3.06 and increase with decreasing (La/Yb)n ratios. Insulation of the conduit walls during ascent of early magmas is considered to have limited the effects of crustal contamination over time.

The Northern Beaulieu River belt overlies the 2.95 Ga plutonic Beniah Complex, ultramafic units, and the 2.9 Ga Beniah Formation quartz arenites. Mafic volcanic facies are massive flows with subordinate pillowed flows and a paucity of fragmental debris, consistent with subaqueous fissure-fed volcanism and local seamount development. A significant mafic dyke complex indicates extension during evolution of the volcanic belt. Felsic volcaniclastic rocks are represented by a succession of planar- and cross-bedded lapilli tuff overlain by wavy- and planar-bedded coarse tuff, grading into planar-bedded fine tuff with black tuff units, overlain by a pillowed flow. The felsic sequence represents shallow-water reworked pyroclastic deposits that accumulated along a shoreface following subaerial eruption. The fining-upward succession capped by a pillowed flow suggests drowning of the sequence. Mafic samples from the Northern Beaulieu River belt resemble NMORB and slightly LREE-enriched EMORB with negative Nb and some negative Ti anomalies. 脦Nd values are between +0.39 and +8.26. One felsic tuff sampled is calc-alkaline.

The Central Beaulieu River belt overlies the 2.95 Ga plutonic Sleepy Dragon Complex and a thin quartz arenite unit. Mafic volcanic facies were deposited on the medial portions of seamounts in water <200 m deep, as indicated by vesicular (5-49%) lobate-pillowed and massive flows, stratified pillow breccia and hyaloclastite. Stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during redeposition of autoclastic hyaloclastite on seamount flanks in shallow water. Felsic volcanic facies include porphyritic flows, flow breccia, disorganized breccia, and lapilli tuff to tuff, which are consistent with the proximal to distal parts of a subaqueous dome-flow complex. Mafic samples from the Central Beaulieu River belt are tholeiitic to calc-alkaline basalts and andesites, are LREE-enriched, and display marked negative Nb and Ti anomalies. Felsic samples are calc-alkaline rhyolites, LREE-enriched, and have large negative Nb and Ti anomalies.

The study areas are best compared with the Cenozoic Japan Sea back-arc, and the Modern Japan arc, Okinawa Trough, and Havre Trough-Kermadec arc-back-arc system in the Western Pacific. These analogues involve continental crust, and are characterized by seamounts, fissure-dominated volcanism, dome-flow complexes, and shallow-water felsic volcaniclastic deposits. Geochemically, the deposits range from tholeiitic basalts to calc-alkaline rhyolites, depending on proximity to the arc. The north-south elongated 2.7-2.72 Ga Kam Group of the Yellowknife belt proper, located approximately 150 km west of the Central Beaulieu River belt and strike-parallel to the three study areas, represents a remnant back-arc basin formed on continental crust prior to the evolution of the 2.67-2.7 Ga belts. Inferred arc migration in the western Slave Province, resulting in an arc to back-arc system that developed over ca. 50 m.y., is comparable to the scale and timing of processes operating in modern arc systems int he Western Pacific, such as Fiji and the Mariana arc.

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