DIFFUSIONAL MASS TRANSFER AND PRE-TECTONIC BANDING IN GREENSCHIST FACIES TURBIDITES

Meta-turbidites in the Pampean Orogen of NW Argentina show a gradational transition over several tens of km from protolith banded sandstones and siltstones into distinctive, uniformly banded, biotite schists. Gently north-plunging, upright chevron folds allow exposure of progressively deeper sections of the Upper Precambrian rocks. The banded schists formed prior to folding and subsequent tectonic cleavage development.

In anchizone regions, banded sandstones contain bedding-parallel clay-rich laminae, 2–5mm-thick, alternating with 1–3cm-thick quartz-rich bands in which graded bedding and inequant, angular detrital clasts are common. Banded sandstones grade into parallel- and cross-laminated siltstones with cm-scale slump folds. These sedimentary structures are preserved within low to middle greenschist facies regions, where banded psammites have regular, cm-scale phyllosilicate-rich (white mica/chlorite) and quartz-rich bands. Metamorphic minerals are larger and more abundant with temperature increase, but graded bedding is locally preserved in quartz-rich bands and cm-scale compositional banding remains subparallel to bedding relicts, even in biotite schists.

Abundant evidence exists for major, pre-tectonic pressure solution enhancement of original sedimentary layering, especially in phyllosilicate-rich bands where mica beards are common and detrital quartz and feldspar grains are significantly flattened parallel to bedding, but with no lattice-preferred orientation or internal strain within quartz grains. Comparative area measurements from low- and high-grade equivalent rocks show that quartz grains in mica-rich bands underwent 26 to 42% volume loss, respectively, whereas in quartz-rich bands volume loss was 16 to 29%, respectively. Our observations indicate that as recrystallization of mica increases with structural depth, dissolution of quartz also increases, favoring passive concentration of mica and enhancing the original sedimentary banding. Thus, the formation of a pseudo-gneissic compositional banding does not require tectonic transposition of an older planar fabric. This model can be applied to the origin and formation of compositional banding in a wide range of meta-sedimentary rocks in other orogens.