Tswaing Crater Confusion
Shortly after the Stone Age Site, the trail reaches a clump of shady trees with some boulders, an ideal spot to have a rest before descending into the crater bowl. After a steep zig-zag sections, the trail passes a site where some lamprophyre rock can be seen, an example of the volcanic rocks at Tswaing that caused confusion about the crater's origin for many years.
Typical lamprophyre occurence on the northern rim.Site 12 represents a particularly well preserved segment of a lamprophyre dyke, which shows very sharp cross-cutting relationship with the granite. Dykes of this rock type were previously cited as evidence for a volcanic origin for the crater and were thought to have a radial or peripheral disposition with respect to the crater centre. Detailed mapping and plotting of the dyke orientations revealed, however, that most of these intrusions have no particular orientation with respect to the crater centre.
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Position of dykes and sills of mafic igneous intrusive rocks from the
Tswaing Crater rim (a) before and (b) after the impact.
The fragmented nature of these rocks indicates that they have been shattered by a recent geological event, most probably by the cratering event. Lamprophyre from Tswaing contains small amounts of iron-magnesium-rich mineral known as biotite. The age of the biotite from Tswaing lamprophyre could be determined as about 1 300 million years, demonstrating unambiguously that these rocks predate the cratering event.
Lamprophyres typically consist of iron- and magnesium-rich minerals. The most abundant (with more than 50%) mineral in the lamprophyre of the Tswaing Crater is diopside (a pyroxene mineral of the composition of CaMgSi2O6). Magnetite is also abundant in these rocks, which gives them a strong magnetic property.
(Taken from Tswaing Meteorite Crater by W U Reinold, D Brandt R de Jongh and J Hancox)
More information regarding Lamprophyres taken from Wikipedia:
Lamprophyre - igneous intrusive rock
Lamprophyres (Greek λαµπρός (lamprós) = "bright" and φύρω (phýro) = to mix) are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.
Petrology
Modern science treats lamprophyres as a grab-bag term for ultrapotassic mafic igneous rocks which have primary mineralogy consisting of amphibole or biotite, and with feldspar in the groundmass.
Lamprophyres are not amenable to classification according to modal proportions, such as the system QAPF due to peculiar mineralogy, nor compositional discrimination diagrams, such as TAS because of their peculiar geochemistry. They are classified under the IUGS Nomenclature for Igneous Rocks (Le Maitre et al., 1989) separately; this is primarily because they are rare, have peculiar mineralogy and do not fit classical classification schemes. For example, the TAS scheme is inappropriate due to the control of mineralogy by potassium not by calcium or sodium.
Genesis
Rock considered lamprophyres are part of a "clan" of rocks, with similar mineralogy, textures and genesis. Lamprophyres are similar to lamproites and kimberlites. While modern concepts see orangeites, lamproites and kimberlites as separate, a vast majority of lamprophyres have similar origins to these other rock types.
Mitchell considered the lamprophyres as a "facies" of igneous rocks created by a set of conditions (generally; late, highly volatile differentiates of other rock types). Either scheme may apply to some, but not all, occurrences and variations of the broader group of rocks known as lamprophyres and melilitic rocks.
Leaving aside complex petrogenetic arguments, it is fair to say that the essential components in lamprophyre genesis are;
- high depth of melting, which yields more mafic magmas;
- low degrees of partial melting, which yields magmas rich in the alkalis (particularly potassium);
- lithophile element (K, Ba, Cs, Rb) enrichment, high Ni and Cr,
- high potassium and sodium concentrations (silica undersaturation is common)
- some form of volatile enrichment, to provide the biotite (phlogopite) and amphibole (pargasite) mineralogy
- lack of fractional crystallisation (generally; there are exceptions)
- high Mg# ( MgO/(FeO + MgO) )
Individual examples thus may have a wide variety of mineralogy and mechanisms for formation. Rock considered lamprophyres to be derived from deep, volatile-driven melting in a subduction zone setting. Others such as Mitchell consider them to be late offshoots of plutons, etc., though this can be difficult to reconcile with their primitive melt chemistry and mineralogy.
Distribution
Lamprophyres are usually associated with voluminous granodiorite intrusive episodes. They occur as marginal facies to some granites, though usually as dikes and sills marginal to and crosscutting the granites and diorites. In other districts where granites are abundant no rocks of this class are known. It is rare to find only one member of the group present, but minettes, vogesites, kersantites, etc., all appear and there are usually transitional forms.
Lamprophyres are also known to be spatially and temporally associated with gold mineralisation. Rock (1991) considered them possible source rocks, but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of the asthenosphere and mantle, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation.
Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands and Southern Uplands of Scotland; the Lake District of northwest England; Ireland; the Vosges Mountains of France; the Black Forest and Harz mountain regions of Germany; Mascota, Mexico; Jamaica and in certain locations of British Columbia, Canada.
To qualify, please answer the following questions:
1. In your own words, shortly describe the confusion that existed regarding the creation of Tswaing Crater.
2. Describe the grain size and colours of the exposed rock.
3. What set lamprophyre apart from most other igneous rock?
4. Where does lamprophyre occur?
5. OPTIONAL: Post pictures of your visit with your log.