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

Compilation of age and endowment data on volcanic-hosted massive sulfide (VHMS), porphyry copper, orthomagmatic nickel, orogenic gold, granite-related rare metal and pegmatite deposits (nearly 1200 deposits from 21 mineral provinces) indicate that metallogenic patterns change over time. For much of Earth’s history, the metallogenesis of convergent margins is marked by a relatively systematic temporal progression of deposits, the convergent margin metallogenic cycle (CMMC): VHMS, calc-alkalic porphyry copper and orthomagmatic nickel → orogenic gold → alkalic porphyry copper, granite-related rare metal and pegmatite. Typically CMMCs last 70-170 Myr, and the progression appears to be related to the convergent margin tectonic cycle (Collins and Richards, 2008). Prior to ~3100 Ma, however, CMMCs are not recognised. Rather, these old mineral provinces are characterised by long metallogenic histories (400-500 Myr) with an irregular distribution of deposits. The Mesoarchean to Mesoproterozoic is characterised mostly by mineral provinces with short (80-150 Myr) metallogenic histories and a single CMMC. Between 1900 Ma and 1800 Ma, however, some mineral provinces (e.g. Trans-Hudson and Sveccofennian) are characterised by multiple CMMCs, with total metallogenic histories that last up to 200 Myr. Paleoproterozoic provinces with multiple CMMCs formed by the consumption of internal seas, whereas mineral provinces on outward-facing convergent margin typically have only one CMMC. After ~800 Ma, convergent margins are mostly long-lived (290-480 Myr) and are characterised by multiple CMMCs with complex metallogenic histories.
The changes in the metallogenesis of convergent margins reflect changes in tectonic processes through time. Prior to 3100 Ma, stagnant lid tectonics, which did not involve subduction, resulted in the formation of oceanic plateaus with irregular periods of mineralisation. After the initiation of subduction at ~3100 Ma, the style of metallogenesis changed. The dominance of provinces with a single CMMC from 3100 to 800 Ma suggests that convergent margins were unstable and could be shut down easily. This is consistent with models of shallow-break-off subduction whereby the subducting slab breaks off at shallow levels due to the lower plate strength in the Archean and the early part of the Proterozoic. When the slab breaks off, the subduction system shuts down and produces a single CMMC. Only in cases where factors such as closure of internal seas force continued subduction do multiple CMMCs occur. The change to longer metallogenic histories and multiple CMMCs at ~800 Ma is likely the consequence of the cooling of the mantle, which increases plate strength, allowing subduction of cold slabs deeper into the mantle and more stable convergence: continuous ridge push and the density of oceanic crust causes re-initiated of subduction further outboard rather than complete termination of subduction when the convergent margin is perturbed by the accretion of an exotic block or other tectonic event. Subduction only terminates upon collision of two major crustal blocks. As a consequence, the metallogenic history or geological young convergent margins is long with multiple CMMIs and/or complex temporal interleaving of deposit types.

Tags: EARTH SCIENCES, Published_External, Wednesday Seminar, metallogeny, techtonics
Organization: Geoscience Australia Data