Nelspruit’s Granite Suite
The oldest magnetite-series granite so far known is the 3105
Ma-old biotite granite of the Nelspruit batholith, South
Africa.
History
First considered as a distinctive granitic unit, the
"Nelspruit-type granite", by Hall (1918), later becoming the
Nelspruit Granite of Visser (1956). Later workers regarded the
granite terrain north of the Barberton greenstone belt as a complex
suite of gneisses and migmatites, and referred to the "Nelspruit
gneisses and migmatites" (Viljoen and Viljoen, 1969). Recognition
of the coeval, batholithic nature of the massive porphyritic
granodiorite/adamellite, the granodioritic gneisses-migmatites and
the fine-grained massive granodiorite came with detailed mapping of
the region (Robb, 1978) and trace element models suggesting
crystallization of a single magmatic entity (McCarthy and Robb,
1978). The name Nelspruit Granite Suite was introduced by Walraven
(1989).
Although the suite is generally well-exposed, the type area is
defined as the tract extending between White River and the
Jamestown Hills, encompassing Nelspruit, where boulder-strewn
kopjes provide excellent exposure of most of the rock types making
up the suite. The granodioritic phase of the suite is best
developed northwest of White River, on the Hebron Forest
Reserve.
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Distribution of the Nelspruit Suite in relation to other major
granite bodies, country rocks and younger cover sequences.
Basic concept and unifying features :
Although the suite occurs as a large, composite batholith, it is
typified by a coarse-grained, porphyritic granodiorite/adamellite,
which grades into granodioritic gneiss and migmatite.
Form and size of intrusion:
Believed to be a large sheet-like intrusion covering an area of
>2000 km².
Petrogenesis:
The enormous volumes of magma required to form the Nelspruit
batholith are believed to have been derived by widespread partial
melting of a pre-existing tonalite-trondhjemite basement (Robb et
al., 1983). Textural and compositional diversity are believed to be
the result of slow cooling which resulted in in situ crystal
fractionation occurring, with K-feldspar phenocrysts nucleating
late in the paragenetic sequence (McCarthy and Robb, 1978). The
magma was emplaced passively and coalesced at a relatively high
crustal level to form a sheet-like body. The migmatites and
gneisses are believed to underlie the sheet-like, porphyritic
granite, and also represent the metasomatized, marginal phases of
the intrusion.
Boundaries :
Southern contact of the Nelspruit batholith with the Jamestown
schist belt is obscured by a well-developed, strongly mylonitized
shear zone. The western edge of the batholith is overlain by
sediments of the Transvaal Supergroup, while the eastern boundary
is defined by the Lebombo Range, comprising of rhyolite rocks. The
northern limit of the suite is poorly defined because of
increasingly poor exposure towards the north. The northern contact
of the Nelspruit Suite should therefore be regarded as
tentative.
Other intrusive subdivisions :
Although the main Nelspruit Suite body is texturally and
compositionally variable, the generally gradational nature of the
transitions from one lithotype to the next indicates that rigorous
subdivisions are inappropriate. The two granodiorite intrusions,
although indistinguishable in age from the main mass, are
sufficiently discrete and texturally different to warrant naming
them as two separate plutons, Hebron and Berlin. The Stentor pluton
closely resembles the Hebron and Berlin plutons which constitute a
phase of the Nelspruit batholith. Thus, it is concluded that the
Stentor pluton also forms an integral part of the batholith.
Other intrusive granites within the boundaries of the Nelspruit
Suite include the Mpakeni, Salisbury Kop and Cunningmoor
plutons.
At least two other composite intrusions of batholithic dimensions
within the Swazian Erathem, the Mpuluzi and Heerenveen Suites, are
recognized to the south of the Barberton greenstone belt.
A batholith (from Greek bathos, depth + lithos, rock) is
a large emplacement of igneous intrusive (also called plutonic)
rock that forms from cooled magma deep in the earth's crust.
Batholiths are almost always made mostly of felsic or intermediate
rock-types, such as granite, quartz monzonite, or diorite.
Although they may appear uniform, batholiths are in fact structures
with complex histories and compositions. They are composed of
multiple masses, or plutons, bodies of igneous rock of irregular
dimensions (typically at least several kilometers) that can be
distinguished from adjacent igneous rock by some combination of
criteria including age, composition, texture, or map able
structures. Individual plutons are crystallized from magma that
traveled toward the surface from a zone of partial melting near the
base of the Earth's crust.
For a geographer, a batholith is an exposed area of mostly
continuous plutonic rock that covers an area larger than 100 square
kilometers (40 square miles). Areas that are smaller than 100
kilometers are called stocks. However, the majority of batholiths
visible at the surface (via outcroppings) have areas far greater
than 100 square kilometers. These areas are exposed to the surface
through the process of erosion accelerated by continental uplift
acting over many tens of millions to hundreds of millions of years.
This process has removed several tens of kilometers of overlying
rock in many areas, exposing the once deeply buried
batholiths.
Batholiths exposed at the surface are also subjected to huge
pressure differences between their former homes deep in the earth
and their new homes at or near the surface. As a result, their
crystal structure expands slightly and over time. This manifests
itself by a form of mass wasting called exfoliation. This form of
erosion causes convex and relatively thin sheets of rock to slough
off the exposed surfaces of batholiths (a process accelerated by
frost wedging). The result is fairly clean and rounded rock
faces.
Although batholiths were once believed to extend to unknown
depths, recent studies have shown that many of them have a
thickness of perhaps 10 to 15 km (6 to 9 miles).
Sources:
Geoscience
Britannica.com
To get to this EarthCache you have to park at one of the parking
co-ordinates below. Your decision will determine the difficulty of
the cache. For the first co-ordinates it is 2½ stars and for the
second co-ordinates it is 3½. Both routes however will give you
great sceneries. Just follow the trails and markings to show you
the right track.
To log your find, answer the following questions and
email them to me (do not post the answers in your log):
1. What is the difference between a batholith, pluton and a
stock?
2. The exposed granite rocks were formed and sculpted over the
years by weathering. What kind of weathering or other processes
were the major role players in forming these boulders.
3. What is the approximate size of the natural birdbath near the
co-ordinates?
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location but is only optional.
You can sent me your answers in either Afrikaans or English.