G. R. Dunning

A U/Pb age for the Shetland Islands oceanic fragment, Scottish Caledonides: evidence from anatectic plagiogranites in ‘layer 3’ shear zones

High precision U/Pb data obtained from zircons extracted from plagiogranite within the gabbro unit of the Shetland Islands oceanic fragment of northeast Scotland yield an age of 492 ± 3 Ma. Field relations indicate that the plagiogranites were generated by the partial melting of amphibolitized gabbros within high-temperature shear zones formed due to crustal deformation and fluid infiltration occurring in proximity to a spreading centre. The U/Pb data therefore constrain the crystallization age of the Shetland complex. This age is similar to U/Pb ages obtained from the Leka (497±2 Ma), Karmoy (493 +7 -4 Ma) and Gulfjellet (489±3 Ma) oceanic fragments of the Norwegian Caledonides, and the Pipestone Pond (494 3 -2 Ma) and Betts Cove (489 3 -2 Ma) oceanic fragments of the Canadian Appalachians.

Episodic reactivation of a Late Precambrian mylonite zone on the Gondwanan Margin of the Appalachians, southern Newfoundland

The Grand Bruit Fault Zone of southern Newfoundland is a fundamental structure within Late Precambrian basement on the Gondwanan margin of the Appalachian orogen. Within the fault zone, a sequence of structures documents changes in the sense of ductile displacement from (1) reverse dip slip, to (2) dextral strike slip, to (3) sinistral oblique slip, and, finally, to (4) dextral lateral offsets. Fault movements along this structure were punctuated by emplacement of a variety of plutons and minor intrusions which, when precisely dated, allow these movements to be bracketed at between 571 Ma and 564 Ma, 497 Ma and 427 Ma, 424 Ma and 420 Ma, and 421 Ma and 387 Ma, respectively. The tectonic evolution of the Gondwanan inlier of southern Newfoundland is mirrored, in large part, by the record of mylonite development within the Grand Bruit Fault Zone. These tectonic events are attributable to well-constrained, regional orogenic events of both the Pan-African and Appalachian cycles. Newly formed shear zones in the fault zone reactivate parts of much older faults of similar regional orientation and are, in some cases, kinematically indistinguishable from the ancestral structures. Integration of precise geochronological data with the sequence of overprinted fault structures demonstrates that, although the role of progressive deformation in shear zone development was important, the observed disposition of structures and rock units is primarily a function of polyorogenic accretion. As a multiple-reactivated structural lineament in a Gondwanan basement inlier, the fault zone exerted fundamental control over the tectonic development of the leading edge of the convergent southeast margin of the orogen.