Petrology of Lamproites by Roger H. Mitchell

By Roger H. Mitchell

In this ebook, the 1st devoted totally to the petrology of lamproites and their relationships to different potassium-rich rocks, the target of the authors is to supply a entire serious overview of the incidence, mineralogy, geochemistry, and petrogenesis of the extended family. even supposing lamproites signify one of many rarest of all rock varieties, they're either economically and scientifically very important and we think the time is ripe for a evaluation of the advances made of their petrology over the last 20 years. lots of those advances stem from the popularity of diamond-bearing lamproites in Western Australia and the reclassification of a number of anomalous diamond-bearing kim­ berlites as lamproites. for this reason lamproites, formerly of curiosity basically to a small variety of mineralogists focusing on exotica outdoor the mainstream of igneous petrol­ ogy, became best objectives for diamond exploration on a global foundation. Contemporaneously with those advancements, petrologists learned that lamproites own isotopic signatures complementary to these of midoceanic ridge basalts, alkali basalts, kimberlites, and different mantle-derived melts. those isotopic stories supplied new insights into the long term improvement of the mantle through suggesting that the resource areas of lamproites have been metasomatically enriched in mild infrequent earth and different incompatible components as much as 1-2 Ga ahead of the melting occasions resulting in iteration of the magma.

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The high modal content of phlogopite imparts an extreme ultrapotassic (K20/Na20 molar ratios up to 21) character to these rocks. Micaceous kimberlites exhibit mineralogical and isotopic similarities to some phlogopite-rich lamproites. Mitchell (1986, 1989a) has suggested that the mineralogical, isotopic, and compositional differences between the two groups of kimberlites are so distinctive that a good case can be made for considering each group to be a distinct magma type, unrelated to each other or to lamproites.

The Shoshonite Association The term shoshonite was coined by Iddings (1895) for orthoclase-bearing alkaline rocks from Wyoming. These are essentially basaltic rocks composed of olivine and augite phenocrysts set in a groundmass of orthoclase-rimmed labradorite, olivine, and leucite. With increasing modal amounts of olivine, they are gradational into absarokite and, with increasing silica into banakite. Joplin (1968) extended the usage of the term by referring to a shoshonitic association of magma series for a suite of basaltic to trachytic rocks that she considered as the potassium-rich equivalent of the alkali basalt series.

Niggli mg = 45-85. Niggli k > 70 (lower values may result from alteration). Typically FeOT and CaO < 10 wt %. High Ba contents (typically> 2000 and commonly> 5000 ppm) coupled with high Ti0 2 (1-7 wt %). Typically Zr > 500 ppm, Sr > 1000 ppm, La > 200 ppm. The majority of olivine/madupitic lamproites fall in the compositional range 40-51 wt % Si0 2, 1-5 wt % Ti0 2, 3-9 wt % A1 20 3, 5-9 wt % FeO (total), 4-13 wt % CaO, 12-28 wt % MgO, < 2 wt % Na20, 3-9 wt % K 20, 1-3 wt % P20 S ' 2-8 wt % H 20 (composition recalculated to 100% on a CO 2-free basis with total iron expressed as FeO).

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