Convergent margin metallogenic cycles: A window to secular changes in Earth's tectonic evolution

Abstract

Compilation of age and endowment data for deposits that commonly occur on convergent margins (volcanic-hosted massive sulfide, porphyry copper, orogenic gold, granite-related rare metal and pegmatite deposits: over 1000 deposits from 21 mineral provinces) indicate that metallogenic patterns have changed over time. For much of Earth's history, metallogenesis along convergent margins is marked by a relatively systematic temporal progression of deposits succeeding one-another, which we refer to as the convergent margin metallogenic cycle (CMMC): volcanic-hosted massive sulfide (VHMS) and/or calc-alkalic porphyry copper → orogenic gold → alkalic porphyry copper, granite-related rare metals and/or pegmatite. Typically individual CMMCs last for 60–160 Myr, and the progression appears to be related to the convergent margin tectonic cycle. Prior to ca. 3000 Ma, however, CMMCs are not recognized. Rather, these old mineral provinces are characterized by long metallogenic histories (370–500 Myr) with no discernible pattern of deposit types. The Mesoarchean to Paleoproterozoic is characterized mostly by mineral provinces with relatively short (60–155 Myr) metallogenic histories and a single CMMC. Between 1950 Ma and 1700 Ma some convergent margin mineral provinces (e.g. Trans-Hudson and Svecofennian) are characterized by multiple CMMCs, with metallogenic histories that last up to 160 Myr. Between 1250 Ma and 750 Ma, longer-lived yet relatively poorly-constrained metallogenic histories (up to 320 Myr) appear, and after ca. 750 Ma, convergent margins are mostly long-lived (290–450 Myr) and are characterized by multiple CMMCs with complex metallogenic histories.These four periods in the metallogenesis of convergent margins appear to reflect secular changes in tectonic processes. Prior to ca. 3200–3000 Ma, stagnant lid tectonics, which did not involve modern-style subduction, dominated, resulting in non-cyclical mineralization. After the initiation of some early form of subduction between ca. 3200 Ma and ca. 3000 Ma, the metallogenic style changed. The dominance of provinces from 3000 to 1700 Ma with a single CMMC, and a relatively short metallogenic history suggests that convergent margins were shorter-lived. This is consistent with models of shallow-break-off subduction whereby the subducting slab breaks off at shallow levels due to lower plate strength beginning in the later Archean. We suggest that between ca. 3000 and ca. 1700 Ma a propensity for slab break-off could shut down individual subduction systems and produce short-lived metallogenic histories with a single CMMC. The change to longer metallogenic histories and dominant multiple CMMCs begins with Rodinia assembly: the length and complexity of metallogenesis systematically increases thereafter. The lengthening of convergent margin metallogenesis resulted from more stable convergence as continuous ridge push and the stronger density contrasts of the subducting slab causing re-initiation of subduction outboard rather than complete termination of subduction when the convergent margin was perturbed. As consequence of these driving factors, the metallogenic history of young convergent margins involves multiple CMMCs and/or complex temporal interleaving of deposit types.