SECULAR CHANGE IN GLOBAL TECTONICS SINCE THE NEOARCHEAN ERA: A NEW PERSPECTIVE FROM METAMORPHIC ISOGRAD SEQUENCES

Regionally metamorphosed rocks provide a direct record of past tectonic activity. Pressures, temperatures, and timescales of metamorphism in the rock record vary as functions of age. With the caveat of imperfection in the rock record, these differences challenge the assumption that Earth’s tectonic processes have always operated as they do today. Here, we present a dataset of metamorphic field gradients and their ages to evaluate secular change in metamorphic processes. Entries are classified into one of six categories, defined by isograd sequences in metapelitic rocks: andalusite–sillimanite, andalusite–staurolite–sillimanite, staurolite–sillimanite, triple-point, kyanite–sillimanite, and kyanite only.

Although the oldest regionally metamorphosed supracrustal sequence (the Isua Greenstone Belt) has kyanite, Archean kyanite–sillimanite field gradients are rare. As the largest contiguous Archean crustal fragment, the Superior Province is an illustrative example. Kyanite–sillimanite isograd sequences are known from two localities: the Pontiac Subprovince of southern Quebec and a small area of the Quetico Subprovince, along the USA–Canada border, west of Lake Superior. The aerial extent of these kyanite–sillimanite field gradients is < 1% of the Superior Province; the rest of the Superior Province has andalusite(–staurolite)–sillimanite field gradients. Other Archean provinces are similar. This contrasts strongly with younger orogenic belts, in which both kyanite–sillimanite and andalusite–sillimanite isograd sequences are abundant. The increase in the relative abundance of kyanite–sillimanite field gradients through time mirrors the increase in the abundance of high-pressure metamorphic rocks (high-pressure granulite, eclogite, and blueschist). These observations suggest that there was less variability among Archean metamorphic/tectonic settings compared to today, supporting a hypothesis of secular change in Earth’s orogenic processes.