Elemental, radiogenic isotope (Sr, Nd) and stable isotope (O) data from samples of the 456 Ma Caledonian Glen Dessarry syenite are all compatible with a two-stage crystal-fractionation model for the evolution of the magmas, locally modified by contamination with a partial melt of the surrounding Moine metasediments. The early-crystallizing phases which controlled melasyenite-leucosyenite evolution were biotite, calcic clinopyroxene plus minor apatite and titanite, whilst in-situ crystal-liquid separation also involved alkali feldspar and amphibole with minor allanite, zirconolite and zircon. The diverse accessory mineral suite controlled significant variations in many of the ‘incompatible’ elements, including inordinate Zr/Hf fractionation. Crustal contamination led to severe disturbance in abundances and inter-element ratios of U, Th; Zr, Hf; Nb, Ta; heavy REEs, Sr and Ba. The elemental characteristics of the magmas are typical of shoshonitic subduction-related suites, with high LILEs, relatively low HFSEs and pronounced negative Nb-Ta anomalies. Inferred isotopic characteristics of the parent magmas ( 87 Sr/ 86 Sr ≈ 0.7040, ΣNd = + 2.5 and δ l8 O ≈ + 7%o) are probably derived from a multicomponent source region involving a substantial input from contemporary depleted mantle.
The formation and reworking of the continental crust is of great importance for understanding the early evolution of the Earth. Combining U−Pb/Hf isotopes in zircons with whole rock geochemical and Sr-Nd isotopic data can provide new insights on petrogenetic mechanisms, timing of magmatic events, crust-mantle interactions and magma sources for crustal material. Here we present a combined dataset of in situ zircon U−Pb and Hf as well as whole-rock Sr and Nd analyses for Archean TTGs and geochemically variable high-K granitoids from the Bundelkhand Craton, India. U−Pb zircon ages reveal that the TTGs were emplaced at 3.42 Ga, 3.33 Ga and 2.72 Ga, while the high-K granites, including sanukitoids, were emplaced between 2.57 Ga and 2.54 Ga. The high-K granitoids have higher initial 87Sr/86Sr isotope ratios than the TTGs. They also display a lower range in initial eNd and eHf values (from -8 to -1 and -8.9 to +0.4, respectively) compared with the TTGs (from -4 to -1.2 and -1.6 to +4.4, respectively). The Hf depleted mantle model ages calculated for high-K granitoids are 3.19−2.86 Ga and for TTGs 3.71−3.48 Ga. The U−Pb ages and chondritic to superchondritic eHf values of the TTGs provide evidence for a long-term episodic growth of juvenile crust from depleted mantle reservoirs between 3.4 and 2.7 Ga. The strongly negative eNd and eHf values of the high-K granitoids, together with geochemical features (variable compatible and incompatible elements) indicate that they were a result of multi-stage reworking of the Paleo- to Neoarchean crust and mixing with magmas extracted from an enriched mantle source during a relatively short-lived tectonic event at the end of the Archean.
The granitoid record has evolved through geological time from tonalite-trondhjemite-granodiorite (TTG) in the Archean toward typical granodiorite and granite with arc magma compositions in the present-day. However, many fundamental questions remain regarding the geodynamics of the early Earth and the timing of the transition to modern subduction-driven tectonics. The Archean rock record is often hampered by metamorphic overprinting and focusing on accessory minerals that may be impervious to metamorphism is a potential promising approach to gain insights into early Earth processes.. Here we present new geochemical data on two accessory phases (titanite, apatite) from a compilation of granitoids that cover a large span of the geological record from the Archean to the Phanerozoic. We demonstrate that trace element analysis and detailed petrographic work can give direct information about the petrogenesis of the host magmas even when these granitoids have been affected by metamorphism. Our results show that the chemical signature of accessory minerals allow discrimination of the different magma types generated during crustal evolution and give insights into their source characteristics. Since these accessory minerals often survive sedimentary processes, the Archean sedimentary record may offer an untapped archive of critical information.
Gabbros drilled from the shallow (720 m) east wall of the Atlantis II transform on the Southwest Indian Ridge (SWIR; 32°43.40',57°16.00')provide the most complete record of the stratigraphy and composition of the oceanic lower crust recovered from the ocean basins to date.Lithologies recovered include gabbro, olivine gabbro, troctolite, trondhjemite, and unusual iron-titanium (FeTi) oxide-rich gabbro containing up to 30% FeTi oxides.The plutonic rock sequence represents a tholeiitic fractionation trend ranging from primitive magmas having Mg numbers of 67 to 69 that fractionated troctolites, to highly evolved liquids that crystallized two-pyroxene, FeTi oxide-rich gabbros and, ultimately, trondhjemite.Isotopic compositions of unaltered Leg 118 gabbros are distinct from Indian Ocean mid-ocean ridge basalts (MORB) in having higher 143 Nd/ 144 Nd (0.51301-0.51319)and lower 206 Pb/ 204 Pb values (17.35-17.67); 87Sr/ 86 Sr values (0.7025-0.7030) overlap those of SWIR basalts, but are generally lower than MORBs from the Southeast Indian Ridge or the Rodrigues Triple Junction.More than one magma composition may have been introduced into the magma chamber during its crystallization history, as suggested by the higher 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, and lower 143 Nd/ 144 Nd values of chromium-rich olivine gabbros from the bottom of Hole 735B.Whole-rock gabbro and plagioclase mineral separate 87 Sr/ 86 Sr values are uniformly low (0.7027-0.7030), irrespective of alteration and deformation.By contrast, 87 Sr/ 86 Sr values for clinopyroxene (0.7025-0.7039) in the upper half of Hole 735B are higher than coexisting plagioclase and reflect extensive replacement of clinopyroxene by amphibole.Hydrothermal veins and breccias have elevated 87 Sr/ 86 Sr values (0.7029-0.7035) and indicate enhanced local introduction of seawater strontium.Oxygen-and hydrogen-isotope results show that secondary amphiboles have uniform δD values of -49 to -54%« and felsic hydrothermal veins range from -46 to -77%o.Oxygen-isotope data for secondary amphibole and visibly altered gabbros range to low values (+1.0-+5.5%c),and O-isotope disequilibrium between coexisting pyroxene and plagioclase pairs from throughout the stratigraphic column indicates that seawater interacted with much of the gabbro section, but at relatively low water/rock ratios.This is consistent with the persistence of low 87 Sr/ 86 Sr values, even in gabbros that were extensively deformed and altered.
The formation and reworking of the continental crust is of great importance for understanding the early evolution of the Earth. Combining U−Pb/Hf isotopes in zircons with whole rock geochemical and Sr-Nd isotopic data can provide new insights on petrogenetic mechanisms, timing of magmatic events, crust-mantle interactions and magma sources for crustal material. Here we present a combined dataset of in situ zircon U−Pb and Hf as well as whole-rock Sr and Nd analyses for Archean TTGs and geochemically variable high-K granitoids from the Bundelkhand Craton, India. U−Pb zircon ages reveal that the TTGs were emplaced at 3.42 Ga, 3.33 Ga and 2.72 Ga, while the high-K granites, including sanukitoids, were emplaced between 2.57 Ga and 2.54 Ga. The high-K granitoids have higher initial 87Sr/86Sr isotope ratios than the TTGs. They also display a lower range in initial eNd and eHf values (from -8 to -1 and -8.9 to +0.4, respectively) compared with the TTGs (from -4 to -1.2 and -1.6 to +4.4, respectively). The Hf depleted mantle model ages calculated for high-K granitoids are 3.19−2.86 Ga and for TTGs 3.71−3.48 Ga. The U−Pb ages and chondritic to superchondritic eHf values of the TTGs provide evidence for a long-term episodic growth of juvenile crust from depleted mantle reservoirs between 3.4 and 2.7 Ga. The strongly negative eNd and eHf values of the high-K granitoids, together with geochemical features (variable compatible and incompatible elements) indicate that they were a result of multi-stage reworking of the Paleo- to Neoarchean crust and mixing with magmas extracted from an enriched mantle source during a relatively short-lived tectonic event at the end of the Archean.
Underexplored accessory minerals such as titanite and apatite have the potential to give insights into the nature and the petrogenesis of their host rock.Their trace element and REE-rich compositions carry a record of crystallisation history and chemical characteristics of their source.Moreover, titanite and, to a certain extent, apatite are resistant to erosion during sedimentary cycles which makes them ideal to reconstruct the history of long-eroded continental landmasses.Here we report new trace element data on apatite and titanite from granitoids of different Archean cratons and comparative granitoids from the Phanerozoic.Trace element signatures of both minerals reveal systematic chemical trends in Y, LREE and Sr contents related to the nature of their host magma, which are used to construct discrimination diagrams delineating Archean TTGs from sanukitoids, and modern adakites from S/I-type granites.By comparing Archean granitoids (TTG and sanukitoids) and their Phanerozoic counterparts (adakite and high Ba-Sr granites), we show that the robust nature of these phases makes them reliable recorders of petrogenetic information from Archean rocks, that usually have been affected by secondary processes (metamorphism, deformation, hydrothermal activity).Applied to the rock record, both phases potentially provide detailed archives of magmatic evolution through time.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
