| There is a close relationship between periods of orogenic metamorphism imprinted in the
rock record and the supercontinent cycle. For example, geologic and paleomagnetic data
support a loosely knit amalgamation of the continental lithosphere in Rodinia ca. 1100–1000
Ma, but by ca. 900–800 Ma this supercontinent had begun to disintegrate; Gondwana was a
product of the first stage in the Rodinia-to-Pangea supercontinent cycle. This cycle involved
the diachronous break-up of Rodinia by rifting and formation of ocean basins along internal
spreading margins while subduction continued at external convergent margins, followed by
the multistage amalgamation of dispersed continental fragments by closure of both internal
and external ocean basins. The network of Brasiliano–Pan-African belts reflects the way part
of the Rodinian continental lithosphere broke up into fragments cored by cratons and how
these fragments were reconfigured in Gondwana. Although the suturing of West
Gondwana was not completed until the Cambrian, it nevertheless formed the nucleus to
which fragments of East Gondwana were accreted during the Ediacaran–Cambrian.
Eclogite–high-pressure granulite metamorphism (e.g., Brazil, Africa) and granulite–ultrahigh
temperature metamorphism (e.g., Brazil, Africa, Madagascar, southern India, Sri Lanka,
and East Antarctica) are characteristic of the network of Brasiliano–Pan-African
orogenic belts. For the lower crust to achieve ultrahigh temperatures (> 900°C) at a
regional scale either there must be an external source of heat, or the crust must
be enriched in the heat producing elements relative to average continental crust
and the duration of the orogenic cycle must be long enough to allow conductive
heating. In both cases heating must overcome the thermal buffering due to partial
melting. These conditions were commonly satisfied during suturing of the continental
fragments making up Gondwana, as evidenced by the widespread formation of
ultrahigh temperature granulite terranes. Although eclogites are generally scarce, the
Trans-Saharan segment of the Pan-African records the oldest coesite-bearing eclogites, and
sutures within the Anti-Atlas and the South China block record the oldest blueschist
metamorphism, features that signal a change to colder thermal regimes associated with
orogenesis during the Phanerozoic. In contrast, for the Laurasian fragments of Rodinia
the first step in the transformation to Pangea involved the successive separation of
Siberia, Baltica and Laurentia from Pannotia by generation of the new lithosphere of
the Iapetus and Rheic ocean basins, leaving a unified Gondwana. Terrane export
across and sequential closure of these ocean basins led to the formation of the large,
essentially linear Appalachian-Caledonian–Variscan–Altaid orogenic system which
sutured Laurasia to Gondwana forming Pangea. The Paleozoic sutures are marked
by high pressure to ultrahigh pressure metamorphism and eclogite–high-pressure
granulite metamorphism, whereas ultrahigh temperature metamorphism is rare. Limited
subduction and the choking of subduction zones by terrane accretion may have
reduced transport of water into the mantle wedge, suppressed the development of
small-scale convection and the formation of backarcs and facilitated the formation
and exhumation of ultrahigh-pressure metamorphic terranes. The large, essentially
linear Terra-Australis accretionary orogenic system formed along the Panthalassan
margin of Gondwana during the Paleozoic and was the precursor of the modern
circum-Pacific orogenic system. The formation of Gondwana coincided with a transition in
global geodynamics to an Earth system dominated by two mantle convection cells, a
Panthalassan cell containing only oceanic plates and a Pangaean cell containing the
continental plates, which controlled the change in scale and style of orogenic systems
formed during the Phanerozoic compared with those formed during the Proterozoic. |