Anorogenic Ring Complex Clusters

Lobo-Guerrero S. Alberto

LOGEMIN S.A., Bogota, Colombia ageo@logemin.com

An anorogenic ring complex cluster (ARCC) is a midalkaline to alkaline batholithic sized body of plutonic and coeval volcanic rocks. The surface expression of a typical ARCC is a 30º-60º-90º scalene triangle whose hypotenuse measures ~340 km and is limited by major, crustal-scale fractures. Over 400 ARCCs have been identified in all continents. They formed during the last 75% of Earth’s geological history and many have associated major volumes of economic mineralization. Understanding ARCCs may lead to the discovery of new mineral deposits.

ARCCs are believed to form by amalgamation of multiple ring complexes of varying composition and dimension that may intersect each other. At any given time, only a few, widely separated complexes are being formed beneath volcanic edifices. The result is a three-dimensional network of tightly welded plutons, dikes, sills and volcaniclastic material. Varying degrees of erosion expose ring complexes in the roots of the edifices, or volcaniclastic rocks on their surface. The youngest ARCCs lie under active volcanic fields in intracontinental rift zones, that have not been eroded deep enough to expose their roots.

Volcanic and plutonic rocks are mafic, ultramafic, undersaturated, and alkaline. These include carbonatites, kimberlites, lamprophyres and anorthosites. An ARCC cycle forms in ~135 million years. Minor precursor and late magmatic events may also occur. ARCC cycle overprinting is often observed. Temporal spacing between repeat cycles ranges from 2190 to 5 million years. Up to five overprinted independent cycles have been identified. An ARCC can be recognized by the occurrence of at least one composite series of magmatic events lasting ~135 million years, scalene triangular geometry of the right dimensions, suitable range of TAS geochemistry, and the occurrence of multiple ring complexes.

Predominant rock types, series of geochronological data, and geological maps are main tools for ARCC identification. Major oxide chemical data are plotted on the modified Total Alkali versus Silica (TAS) diagram; radiometric ages are plotted on an event diagram; the surface outline of the cluster defines its size and shape. ARCCs display multiple rock variations.

Techniques to define ARCCs in a given region are: SHRIMP dating clastic zircons in sediments derived from potential clusters, and airborne geophysics to identify triangular bodies of the correct size displaying internal ring structures.

Many ARCCs are composed by metamorphic rocks. Intense sodic alteration, hematitization, potassic enrichment, Na-K depletion, and ultra-high pressure metamorphism are among the varied features of regional metamorphism observed. Incipient migmatitization and alteration of ARCC rocks modify their chemistry to a point where traditional geochemical means are not effective to establish their environment of emplacement.

ARCCs host a significant portion of the world’s mineral deposits, including various precious and base metals, rare metals, building stone, industrial materials and nuclear fuels. Many iron oxide-copper-gold provinces are closely tied to ARCCs. Among these are Carajás, Brazil’s most important mineral district, and the Australian Cloncurry Basin and Stuart Shelf District. Other relevant mineral provinces are the Sulu Peninsula in Jiaodong, which is China’s premier gold producing district, and the Colorado Mineral Belt in the U.S.A. ARCCs host some of the richest diamond-bearing kimberlites. The planet’s largest gold accumulations, including the Witwatersrand Basin, are probably ARCC related. Five ARCC cycles lie in the basement to the Zambian Copperbelt.

ARCCs most likely form under aulacogens or abandoned triple-point rift arms. The widely varying rock types could be produced by mixing of magma from the mantle and recycling of crustal material. Plutonic ring complexes form 3 to 4 km beneath the surface volcanic rocks. Quaternary analogues occur in the rifts of Eastern Africa; the Oslo Graben represents a further stage, where erosion partly removed surface volcanics exposing plutonic rocks. Rift-related large lakes display close associations with ARCCS. Clusters form particular pensinsulas like the Sinai Peninsula. A few oceanic islands present most features described for ARCCs; their genesis could be related.

Many old ARCCs preserve their characteristic triangular shape. This implies that the clusters are not lost to subduction processes and behave as relatively undeformable, rigid crustal fragments during compression and shearing. ARCC rocks remain warm for long periods due to high heat flow in their source area, multiple intrusions over 135 million years, and the fact that part of them are long-lived high heat producers.

Clusters of up to ten ARCCs have been identified in several parts of the world. These seem to occur by tectonic amalgamation in mobile belts. Geological processes that group ARCCs are not well understood. ARCC grouping is perceived to be one of the principal ways to produce continental crust.