I am studying two late Cretaceous K-rich granitoid intrusions, including a charnockite and a high-K calc-alkaline granite. These two intrusions are very near to each other in spatial distribution and nearly coeval in formation time.
The samples from the high-K calc-alkaline granite have zircon ƐHf(t) values generally similar to those of the charnockite samples. Further geochemical evidences with respect to whole-rock and mineral composition indicate that the high-K calc-alkaline might have been formed by fractionation of the primary charnockite magma. For example, relative to the charnockite intrusion, the samples from the high-K calc-alkaline granite display significantly higher SiO2 centents and A/CNK values, indicating that the latter were produced by fractional crystallization from the former. In the Harker diagrams, samples from the high-K calc-alkaline granite and the charnockite intrusion exhibit relatively well-defined linear trends, with the former having significantly lower Al2O3, CaO, TFe2O3, MgO, P2O5, TiO2, Sr, Ba, Eu, Cr and Ni as well as higher Rb and K2O concentrations relative to the latter. This implies that these two intrusions most likely belong to comagmatic evolution product, but the high-K calc-alkaline granite has a higher degree of differentiation than the charnockite intrusion.
Common senses in terms of petrogenesis tell us that, in a comagmatic assemblage, the late-stage highly fractionated rocks usually have higher total REE concentrations and greater (La/Yb)N ratios (i.e., more enriched LREE relative to HREE) than the early-stage less evolved rocks. However, it is not the case for our studies. Although the samples from the high-K calc-alkaline granite have similar REE and trace element patterns to the charnockite intrusion, the former have lower total REE concentrations and are less enriched in LREE than the charnockite. So, I want ask whether this observation is contrary to the petrological common senses, and whether the high-K calc-alkaline granite can be interpreted to be a fractionation product from the primary charnockite magma?
A Fig. showing REE and trace element patterns are attached as a file here. The red lines refer high-K calc-alkaline granite while the black lines to charnockite intrusion.
With best regards.
Lu
This is correct. If the fractionating magma precipitates LREE-concentrating accessory minerals, such as allanite and xenotime, the subsequent comagmatic intrusion will be LREE depleted. The story is more complicated with the Y+HREE, because even if xenotime is fractionating, the newly crysatllized magma may not necessarily be depleted in those elements. The only fact which is not in accordance with a comagmatic origin, is the lower concentration of Rb. Did the younger phase experienced alteration of micas?
Dear everybody: this sentence written by Lu-Lu I think it was misinterpreted. He stated:
"the samples from the high-K calc-alkaline granite display significantly higher SiO2 centents and A/CNK values, indicating that the latter were produced by fractional crystallization from the former. In the Harker diagrams, samples from the high-K calc-alkaline granite and the charnockite intrusion exhibit relatively well-defined linear trends, with the former having significantly lower Al2O3, CaO, TFe2O3, MgO, P2O5, TiO2, Sr, Ba, Eu, Cr and Ni as well as higher Rb and K2O concentrations relative to the latter".
If we analyze it carefully, and susbtitute all this "the former and the latter" way of expressing, it can be restated more clearly as follows:
"the samples from the high-K calc-alkaline granite display significantly higher SiO2 centents and A/CNK values, indicating that the granite was produced by fractional crystallization of the charnockite. In the Harker diagrams, samples from the high-K calc-alkaline granite and the charnockite intrusion exhibit relatively well-defined linear trends, with the granite having significantly lower Al2O3, CaO, TFe2O3, MgO, P2O5, TiO2, Sr, Ba, Eu, Cr and Ni as well as higher Rb and K2O concentrations relative to the charnockite".
This means that the granite has more Rb and K than the charnockite, and Dr. Forster observation is no longer valid, as this is a perfectly normal fractionation sequence. But what doesn't convince me of Lu Lu's interpretation is: How the fractionation of an almost anhydrous magma, such as the one which formed the charnockite, could give rise to an quite hydrated calc-alkaline granitic rock?
With regards, Sebastian.
Dear Lu Lu.
Two magmatic suites were emplaced during the post-collisional evolution of orogens (Vander Auwera et al., 2010): an Anorthosite–Mangerite–Charnockite suite (AMC), and an hornblende- and biotite-bearing granitoids suite (HBG suite).
The mafic facies of the AMC and HBG suites have similar geochemical compositions. Both suites display similar extent of differentiation. However, the HBG suite is hydrous and has a broadly higher fO2 whereas the AMC suite is anhydrous (Frost and Frost, 2008). Therefore, I agree with Sebastian that the granitic facies was not derived by fractionation from the charnockitic one. In spite of this there is an example where both suites display mingling relationships (Vander Auwera et al., 2014).
References:
Article Sveconorwegian massif-type anorthosites and related granitoi...
Article On charnockites
Article Source-derived heterogeneities in the composite (charnockite...
Dear Roman Terentiev: I have no idea where Voronezh State University, but somehow it "sounds" as it could be in the Ukraine. Am I right? You probably won't believe this, but we have in northern Venezuela some small bodies (2-3 km long), tectonically emplaced in the Caribbean Range, of an AMC Suite which has the same U-Pb in zircon age (920 Ma) as the Norwegian Rogaland massif, and that even hosts some Ti-Fe-oxide magmatic segregations! It is called Yumare Complex. Due to its tectonic emplacement in a low grade environment, the rocks are quite altered and sheared, but with some careful petrogrpahic study I was able to identify mafic to intermediate charnockites, mangerites and anorthosites. A similar anorthositic massif crops out in the SE flank of the Sierra Nevada of Santa Marta, in Colombia. These bodies, together with the larger Oaxaca Massif in southern Mexico, represent RODINIAN blocks left over in the America's when Pangea was disrupted in the Jurassic. In oil well cores drilled some 150 km NW of Yumare this AMG Suite has been found associated with a granulitic belt, which also crops out some 50 km south of the Yumare Complex, forming an E-W elongated 4 km hill: El Guayabo Complex. The model proposed to explain these Neoproterozoic high-grade bodies in Southern and Central America, implies that the Amazon Craton collided in one of its margins with the Baltic Shield, and in another margin with the Canadian Shield, some 1.1-1.0 Ga ago, as part of the final amalgamation of the Rodinia Supercontinent, and that a post-orogenic/post-collisional AMG Suite was intruded some 920 Ma ago in the roots of the collapsed orogenic belt! This belt has been called the Putumayo Orogen, and it represented an almost orthogonal branch of the main Grenville-Sunsas Orogen, which crops out in NE North America and SW Brazil. Surely the world map was quite different in Neoproterozoic times than it is today! I will download the three papers you adjoined about charnockites and will read them shortly. Regards, Sebastian.
Dear Sebastián Grande
Thank you for your response and explanations. Your tectonic construction on the Rodinia supercontinent can be viable. The AMC and HBG suites are found on various continental blocks from Neoarchean to Neoproterozoic. Therefore, your find (Yumare Complex) is quite obligate.
I am from Russia
Best wishes,
Terentiev Roman
Dear Dr. Förster
Thank you for your response. The more evolved "granite" (as Sebastián Grande' revision above) is not depleted but slightly enriched in Y+HREE than the charnockite. It has no obvious mica alteration.
In thin sections, we observed some acessary minerals in the granite, including titanite, epidote, magnetite, zircon and miner apatite. Moreover, some samples from the granite have K-feldspar phenocryst indicating fractional crystallization of K-feldspar. This process can lead to the depletion of Rb?
Thanks.
Lu
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