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REPORT

Processes Controlling the Growth and Evolution of Continental

Batholiths, Coast Mountains, British Columbia, Canada

Terrace, British Columbia, Canada
12–17 August 2018

CONVENERS SUMMARY OF EXISTING DATA
Harold Stowell, Geological Sciences, University of Alabama, The Coast Mountains batholith includes 170 to 45 Ma plutons
Tuscaloosa, Alabama, 35487-0338, USA, hstowell@ua.edu that vary from gabbro to leucogranite that intruded host rocks
Margaret Rusmore, Dept. of Geology, Occidental College, from two composite terranes (e.g., Cecil et al., 2018). These rocks
Los Angeles, California 90041, USA, rusmore@oxy.edu have provided stimulus for numerous geological research and
Glenn Woodsworth, Geological Survey of Canada, Vancouver, mapping projects, including the pioneering maps produced by the
British Columbia V6B 5J3, Canada, tricouni@telus.net Geological Survey of Canada, particularly by the late W.W.
Hutchison (1982) and J.A. Roddick (1970). These maps delineated
INTRODUCTION the fundamental contacts for the Coast Mountains batholith and
This Thompson Field Forum brought together 27 geoscientists the tectonostratigraphic framework that the plutons intruded. The
in Terrace, British Columbia, Canada, to discuss the processes maps served as the foundation for decades of research, led chiefly
responsible for production of batholiths in continental magmatic by Lincoln Hollister and Maria Crawford, who guided numerous
arcs. The forum focused on how the existing petrologic, geo- projects with students, post-docs, and other colleagues. Their
chronologic, and structural data for the Coast Mountains batholith work led to fundamental advances in knowledge of granulite
can inform modern paradigms for batholith growth and magma metamorphism, crustal thickening during batholith growth, the
genesis. Decades-old models based on a robust but limited dataset production of batholith melts, and collapse of thickened crust
along the well-exposed Skeena River corridor between Terrace during the last stages of batholith growth.
and Prince Rupert provide a unique opportunity to test modern Geological (e.g., Crawford et al., 1987) and geophysical stud-
orogenic models using new techniques and expanded datasets. ies (e.g., Morozov et al., 1998) outline the crustal architecture of
the Coast Mountains batholith near Terrace–Prince Rupert. This
OVERVIEW architecture includes a fundamental offset of the Moho and
The forum introduced participants to the spectacular geol- prominent structural break known as the Coast shear zone. West
ogy along the Skeena transect across the British Columbia of this shear zone, the crust averages ~26 km in thickness and
Coast Mountains and focused attention on areas where future includes rocks of the Insular superterrane, which includes the
study may resolve outstanding questions. We began with an Alexander, Yukon-Tanana, and Wrangellia terranes. East of this
informal Sunday night meeting at the Thornhill Pub in shear zone, the crust averages 30 km in thickness and includes
Terrace, continued by exploring the low-grade Intermontane rocks of the Intermontane superterrane, here primarily Stikinia.
terrane rocks along the eastern flank of the Coast Mountains Jurassic and Early Cretaceous plutons occur across the batholith
batholith on Monday, examined the Central Gneiss Complex and may represent disparate arcs. By mid-Cretaceous time, how-
and associated plutons on Tuesday, utilized bus and helicopter ever, a single eastward migrating arc was established (Gehrels et
to access the high-grade rocks of the Central Gneiss Complex al., 2009). The Late Cretaceous core of the batholith is east of the
on Wednesday, boarded a boat in order to access islands in the Coast shear zone and includes numerous plutons and the granu-
western metamorphic belt on Thursday, and returned to the bus lite facies rocks of the Central Gneiss Complex. Pioneering work
for stops in the Ecstall pluton on Friday. This final day of the on this granulite delineated the early high P and T conditions
forum for most of the group ended with dinner and an orga- and the partial melt reactions in the Central Gneiss Complex,
nized discussion on the current state of knowledge and future which was rapidly exhumed during the Eocene (Hollister, 1982).
research directions for research on batholith growth and evolu- Partial melting in the Central Gneiss Complex was synchronous
tion. Twelve of the group stayed an additional day and used with intrusion of large plutons (e.g., Kasiks Sill), which include
vans on Saturday for a long trip north to board helicopters and both mantle and crustal signatures. Rapid exhumation of the
fly to the Seabridge Gold KSM property. We were treated to a Central Gneiss Complex is interpreted to have been accommo-
spectacular look at porphyry copper mineralization freshly dated by top-to-the-east detachment faults between about 55 and
exposed by retreating ice along the eastern flank of the Coast 50 Ma. The Shames River detachment is the structurally lowest
Mountains batholith. and most significant shear zone of this system. This detachment

20 GSA Today | March-April 2019