Cogenetic dykes the key to identifying diverse magma batches in the assembly of granitic plutons

Guo, Chun-Li, Wilde, Simon, Henderson, Robert A., Li, Qiu-Li, and Yin, Bing (2020) Cogenetic dykes the key to identifying diverse magma batches in the assembly of granitic plutons. Journal of Petrology, 61 (11-12).

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Abstract

This study demonstrates that dykes that are coeval and cogenetic with plutons can provide an important tool for recognizing discrete batches of magma with similar overall chemical compositions and physical attributes, but different isotopic characteristics, and which contributed to pluton formation. The Qianlishan granitic pluton, located in the Qin–Hang fault zone separating the Yangtze block from the Cathaysia block in South China, was emplaced at 155–152 Ma in the Late Jurassic. It consists of a central zone of strongly differentiated zinnwaldite-bearing equigranular granite surrounded by a less differentiated porphyritic granite. The pluton is spatially associated with an extensive granitic dyke swarm dated here at 153–152 Ma, demonstrating a coeval relationship. Amongst the dykes, two discrete end-member sources can be identified from the bimodal nature of their zircon hafnium and oxygen systematics, with one group showing a range in εHf(t) of -11·9 to -8·0 and in δ18O of 9·0–10·4‰, whereas in the other group the ranges are from -7·3 to -4·1 and 8·4–9·4‰, respectively. This contrasts with the two phases of the Qianlishan pluton, which record wide ranges in εHf(t) of -11·1 to -5·1 and in δ18O of 8·3–10.4‰, but without bimodality. Hence, the overlapping Hf–O isotopic profiling shows the dykes and pluton to be cogenetic. Small-volume magma batches, with their rapid transport through the crust and quick cooling, are all typical features of dyke generation, thus preserving the original heterogeneous Hf–O isotopic signatures that are characteristic of two distinct crustal sources. However, although the pluton was formed from similar sources to the dykes, the bimodal source identity was lost during its assembly through mixing of the magma batches. These findings also provide a potential explanation for the wide range of zircon hafnium isotopic systematics typical of granitic plutons, as shown by sampling at all scales.

Item ID: 67697
Item Type: Article (Research - C1)
ISSN: 1460-2415
Keywords: coeval and cogenetic dykes and pluton; incrementally assembled pluton; zircon Hf–O systematics; Qianlishan pluton; South China
Copyright Information: (C) The Author(s) 2020. Published by Oxford University Press. All rights reserved.
Funders: National Key Research and Development Program of China (NKRDP), National Natural Science Foundation of China (NNSFC)
Projects and Grants: NKRDP [2016YFC0600208], NNSFC 41273043, NNSFC 41773028
Date Deposited: 13 May 2021 00:35
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370503 Igneous and metamorphic petrology @ 100%
SEO Codes: 25 MINERAL RESOURCES (EXCL. ENERGY RESOURCES) > 2503 Mineral exploration > 250399 Mineral exploration not elsewhere classified @ 30%
28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 70%
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