Geology Sonnet 4

Enough with carbon, climate variation

Let’s look at rocks from a far older time,

Which lacked much copper mineralization,

And when anorthosites were at their prime.

Earth’s middle age- boring for a reason?

Tectonics were remarkably unchanged.

Ice and iron were both out of season.

A billion years of uniform exchange

Of isotopes, strontium, and S

The active margins ringed the continent.

Slow, steady mantle cooling caused the process

Strong lithosphere held melts incipient

  It ended with Rodinia dispersion

  Which led to Earth’s exciting, current version.

Peter A. Cawood, Chris J. Hawkesworth. (2014) Earth’s middle age. Geology 42 503-506.

Abstract

Earth’s middle age, extending from 1.7 to 0.75 Ga, was characterized by environmental, evolutionary, and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of preserved passive margins, an absence of a significant Sr anomaly in the paleoseawater record and in the εHf(t) in detrital zircon, a lack of orogenic gold and volcanic-hosted massive sulfide deposits, and an absence of glacial deposits and iron formations. In contrast, anorthosites and kindred bodies are well developed and major pulses of Mo and Cu mineralization, including the world’s largest examples of these deposits, are features of this period. These trends are attributed to a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by ca. 1.7 Ga and continued until breakup of its closely related successor, Rodinia, ca. 0.75 Ga. The overall low abundance of passive margins is consistent with a stable continental configuration, which also provided a framework for environmental and evolutionary stability. A series of convergent margin accretionary orogens developed along the edge of the supercontinent. Abundant anorthosites and related rocks developed inboard of the plate margin. Their temporal distribution appears to link with the secular cooling of the mantle, at which time the overlying continental lithosphere was strong enough to be thickened and to support the emplacement of large plutons into the crust, yet the underlying mantle was still warm enough to result in widespread melting of the lower thickened crust.

See the previous posts in this series for background.

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