Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina
Authors: Cornet Julien , Laurent Oscar , Wotzlaw Jörn-Frederik , Otamendi Juan , Bachmann Olivier 
Published: March 2020

&lt;p&gt;The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fert&amp;#237;l, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust.&amp;#160;&lt;/p&gt;&lt;p&gt;From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of ca. +8 &amp;#8240;; &amp;#949;Hf&lt;sub&gt;t&lt;/sub&gt; of ca. &amp;#8211;3) to a mantle-like signature (granites and rhyolites: zircon &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of ca. +5 &amp;#8240;; &amp;#949;Hf&lt;sub&gt;t&lt;/sub&gt; of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and &amp;#8220;self-shielding&amp;#8221; of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI &gt;1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.&lt;/p&gt;