Zircons recovered from oceanic diorite and gabbro exposed on the northern wall of the central rift valley, SWIR, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. There is little magnetic data for the Gallieni Fracture Zone, so directly comparing zircon U–Pb ages with the magnetic ages is difficult. As a first-order approximation, a half-spreading rate of 4.1 ± 0.4 km/Myr is defined by the zircon age (5.38 Ma for sample D1401) versus distance from the ridge axis (~20 km; an along-axis uncertainty of 2 km is estimated based on the valley bathymetry patterns), much lower than the predicted 5.5 km/Myr on the Antarctic and African flanks of SWIR14. Anomalously older zircon ages than the predicted magnetic ages for a given portion of crust were reported in several recent studies on Atlantis Bank6,7, which were commonly interpreted to reflect assimilation of pre-existing gabbroic rocks from the mantle lithosphere6. Following this interpretation, the age range of our anomalously old samples (1.3 to 1.7 Myr) would reflect crystallization over depth of ~7 to 9 km below seafloor in the axial valley by assuming a constant spreading rate of 5.5 km/Myr. This depth would be reduced significantly when considering the geometry of the fault15,16. Alternatively, the anomalously old zircon may reflect sample spacing due to the uncertainty in location by dredging. This interpretation appears to gain support from the observation that the anomalously old zircon relative to the magnetic age, was limited to surface rocks6,7 and are not found in drill-hole samples in Atlantis Bank8. A similar discrepancy between surface and drill-hole sample occurs at Atlantis Massif in the Mid-Atlantic Ridge, where the surface samples yield an anomalously old age, whereas drill-hole samples did not15,16, supports the latter interpretation. The most likely explanation, however, is that spreading is asymmetric, roughly 10 cm/yr to the south and 4 cm/yr to the north. Similar asymmetries are common along the Southwest Indian Ridge, both by examining isochrons and the EMAG2 magnetic anomaly map20 in the GeoMapApp database. Such asymmetric north- south spreading has been confirmed at Atlantis Bank by paleomagnetics21,22,23 and zircon age dating5,6,8 where the ridge spreads at ~1 cm/yr to the south and 4 cm/yr to the north. This is the first time that zircon dating has confirmed asymmetric spreading away from the transform, as in the other cases it is the spreading direction parallel to the transform where this is documented. More distributed surface sampling combined with drilling will clarify the age discrepancy and provide a more accurate estimate of crustal inheritance.

The relatively young ages for the porous zircons may reflect solid-state recrystallization12,24,25 and/or be related to hydrothermal fluid flow associated with intrusion of felsic veins in this area, suggesting that the porous textures developed soon after crystallization of normal magmatic zircons. The initial 176Hf/177Hf compositions of the normal magmatic grains overlap those of the porous domains, suggesting that fluids catalyzing the formation of the porous zircon domains were derived essentially in situ from the magma that crystallized the zircon originally. The sample has a roughly similar range of Hf-isotope ratios, with the local ridge peridotites along the SWIR19,26, but are more radiogenic than almost all basalts analyzed thus far (except for one basalt from Discovery FZ)19.

The SWIR formed with the Mesozoic breakup of Gondwanaland. Zircons from sample D4-2-3, ~60 km south of the axial valley of the SWIR and immediately to the east of the Gallieni Transform fault, have an unusually old U–Pb age of 180.1 ± 1.0 Ma, which is comparable to the initial timing of the opening of the Indian Ocean. Are these rock/zircons erratic or in-situ? The age similarity between this ancient sample and the Karoo large igneous province magmatism (179–183 Ma)27, which is associated with the breakup of the Gondwanaland at c. 180 Ma, apparently suggest an erratic origin, such as ice-rafting and tsunami-depositing, for the ancient rocks. However, the sample location is far away from the continental margins and it is impossible to sustain the drop-stone theory unless a reasonable explanation is given for the dropping of rocks exclusively on the Gallieni FZ. The sample is thus considered in-situ or only slightly moved in origin (see below for further discussion).

Are these rock/zircons relics or formed in-situ along the spreading axis? If the rocks/zircons were formed during the generation of the magma, their age would be expected to be about ten million years at most. We thus consider two contrasting possibilities for the origin and the route of the old rock/zircons: 1) An intact Gondwana fragment on the sea-floor ever since formation; 2) A partially altered relic that once resided in shallow regions of the upper mantle and was entrained and transported to the vicinity beneath the axis. Zircon, residing in shallow regions of the upper mantle on the order of a hundred million years, would significantly lose its radiogenic lead via diffusion28, protracted residence of the fragments in the upper mantle is thus highly unlikely.

The 176Hf/177Hf ratio of 0.282630 ± 0.7 (2SD) for the whole rock of sample D4-2-3 is higher than those of the zircons (0.282609–0.282547). The zircon initial ε Hf values show a variation from −4.5 to −2.3, which is distinct from those of the SWIR peridotites and basalts (+13.9–+6.4)19. The unradiogenic Nd isotope (ε Nd(t) = −2.4), together with the wide range of initial ε Hf values of zircons, may suggest crustal contamination or interaction between asthenosphere-derived melts and the metasomatized lithospheric mantle in the generation of magmas. The analogous geochronology and isotopic signatures between the ancient diorite and the Karoo magmatic rocks27,29, imply that the diorite may be correlated with the Jurassic magmatic rocks that emplaced prior to the breakup of southern Gondwana and the opening of the SW Indian Ocean. The rock may represent the relicts onto the margin of the southern Gondwana during the rifting of Africa and Antarctica, possibly a mixture of juvenile and recycled crust in a continental magmatic arc30.

Unusually old ages have also been reported from the Mid-Atlantic Ridge3,4. Several possible mechanisms were introduced to explain the origin of these rocks, mostly of continental affinity, such as ceased ocean-floor31, non-spreading blocks1, oscillatory spreading4 and non-drifting slices3. However, all the explanations are difficult to verify and no convincing data for these hypothetic mechanisms have been given so far. Finding an age of 180 Ma in zircons in a quartz diorite on the SW Indian Ridge is likely to raise considerable doubt as to an ocean ridge origin of the sample. At first glance, it would seem most probable that the sample is a glacial erratic deposited on the seafloor on crust of some ~5 Ma age. Several factors make it possible to reconsider this hypothesis.

First is the condition of the rock and state of alteration, which are what would be expected for a highly evolved plutonic rock in this particular tectonic setting. Quartz diorites do occur as late differentiates in oceanic gabbro suites and crosscutting abyssal peridotites. They are not common, but they exist. Highly evolved gabbros and their differentiates are most commonly found as late shallow intrusives into gabbros and peridotites on transform walls where this sample came from, while primitive gabbros are more commonly found near segment centers32. Petrographic examination shows mafic minerals are largely replaced by green amphibole and chlorite, while plagioclase appears albitized (Fig. S1). This suggests an upper greenschist facies assemblage that is typical of late high-level intrusive rocks at ocean ridges due to hydrothermal circulation into the crust and shallow mantle near transforms. Moreover, the rock itself shows no signs of glacial scour or rounding which are common in ice-rafted dropstones deposited on the seafloor. So unlike many dropstones investigated by H. Dick over his years’ survey and sampling along the SW India Ridge, the sample is appropriate to its geologic context.

Next is the age of the sample itself. One hundred and eighty million years coincides with the Karoo volcanic event and the initiation of continental breakup. Most of the borderlands to the SW Indian Ridge along the coastline of South Africa are Archean cratonic rocks. However, on the eastern side of Africa and Madagascar lies rocks of the Neoproterozoic Pan-African Orogenic Belt, with their conjugate in East Antarctica. The Pan African Orogenic Belt is an accreted terrain, assembled during the closure of the Mozambique Ocean during the assembly of Gondwana from old island arc terrains and fragments of reworked Archean cratonic rocks. The Karoo volcanic event was widespread and its remnants are found in South America, Antarctica, Africa and Australia and would have intruded the Pan African Orogenic Belt during Gondwanan breakup.

There is substantial evidence, based on the geochemistry of the peridotites from the Dragon Bone spreading segment, immediately north of that of the dredged diorite, that the present mantle source of the SW Indian Ridge there represents such old arc-mantle wedge mantle drawn up between the plates along the ridge30. The Gallieni FZ nucleated at the margin of a bathymetric plateau lying to the north flanking the eastern side of the Madagascar Plateau. Thus it represents a fracture zone that likely formed by the breakup of an old Gondwanan continental fragment of the Pan-African Orogenic Belt only about 40 to 50 million years ago and is a relatively new and quite shallow ocean basin. Thus, it is reasonable that old Neoproterozoic arc-mantle wedge material, cross intruded by diorite during the initial phase of breakup of Gondwana, was drawn up between the diverging plates and emplaced into the wall of the Gallieni Transform.

Yet to be considered in the discussion is that the quartz diorite is undeformed. Thus, it was not emplaced into its present position in the solid-state, rafted in with delaminated mantle lithosphere. Instead it was intruded into the shallow mantle beneath the rift valley floor, before being unroofed and emplaced onto the transform wall. Mantle peridotites dredged from the Dragon Bone amagmatic segment have universally undergone such deformation and any intrusive within those rocks would have been deformed with them. However, this was not the normal thermal environment of an ocean ridge segment. First, this amagmatic segment exposes an enormous region of mantle peridotite where the sample was taken. Thus there was little volcanism at the time of emplacement of the diorite and enclosing mantle rocks to the seafloor. This can be ascribed to either very refractory mantle source material or to an extraordinarily cool upper mantle due to ultraslow spreading and the cooling effect of the adjacent 110 km offset Gallieni FZ on the upwelling mantle at the ridge transform intersection. Given the already amagmatic state of spreading, this transform edge effect33, would almost totally suppress mantle melting. In such a situation, a quartz-diorite vein, with its very low melting point (~800–1,000 °C) compared to basalt (~1,500–1,250 °C), could be remobilized as the mantle upwelled, while the enclosing peridotite was largely unaffected, to intrude at higher level, carrying its original Gondwanan zircons with it.

This scenario is a reasonable explanation for the emplacement of the quartz diorite, its intrusion into the shallow mantle beneath the rift valley floor, why it is not deformed, but preserves the ancient zircons. It is circumstantial, rather than definitive evidence that permits the possibility that the 180 Ma zircons may have been emplaced in-situ – or not.

Analyses methods and data reduction

Major-element compositions of minerals were analyzed by electron probe microanalysis with a JEOL JXA-8230 Superprobe system at Tongji University. The analyses were performed using an accelerating voltage of 15 kV and beam current of 10 nA, using a 5 μm diameter beam. Natural and synthetic mineral standards were employed for all minerals. JEOL software using ZAF corrections was employed. Representative major-element compositions of feldspar, amphibole, epidote and chlorite are given in Supplemental Table S1.

The U–Pb isotopes for zircons in thin sections were obtained with a Cameca ims1280 microprobe housed at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. The Pb/U calibration was performed relative to the reference zircon Plešovice, which was analyzed repeatedly throughout each session. Operating conditions and analytical protocols are essentially the same as described by Li et al.34. A ~20 μm beam was used. U–Pb data for zircon fragments, extracted from rocks using standard crushing, heavy liquid and magnetic separation techniques and zircon trace element analyses, were obtained by LA–ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, using a pulsed 193 nm ArF Excimer laser coupled to an Agilent 7500 ICPMS with a spot size of 32 μm. Zircon standard 91500 was used to normalize fractionation during analysis. External calibration was performed relative to GJ-1 and NIST 610 combined with internal standardization of Si. Off-line selection and integration of background and analytic signals, time-drift correction and quantitative calibration were performed followed that of Liu et al.35.

A correction for common Pb was made using the 207Pb method36 and an age appropriate model Pb composition37. An additional correction has been made to adjust for initial 238U–230Th disequilibrium using the equations of Schärer38. Zircon commonly hosts ample U relative to Th, which will produce a deficit of radiogenic 206Pb due to an initial 230Th deficit and yield a 206Pb/238U age too young. To correct for these disequilibria for the young sample D1401, the ratio of the mineral/melt partition coefficient for Th and U for zircon (f = [Th/U] zircon /[Th/U] magma ) is estimated by dividing the individual spot analyses with the Th/U ratio of bulk rock analyses for rocks collected in this area (Th/U = 2.6 ± 0.5 (95% confidence); data are compiled from PetDB (http://www.earthchem.org/petdb). Both 206Pb/238U and 207Pb/206Pb ratios are adjusted to account for Th-disequilibria, with these new ratios used to calculate a 207Pb corrected 206Pb/238U age for each analysis of sample D1401.

Zircon Hf isotope analyses were conducted in the the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan, using a Finnigan Neptune plus multi-collector ICP-MS and with a Geolas 2005 excimer ArF laser laser ablation system. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. We applied the directly obtained β Yb value from the zircon sample itself in real-time. The 179Hf/177Hf and 173Yb/171Yb ratios were used to calculate the mass bias of Hf (β Hf ) and Yb (β Yb ), which were normalised to 179Hf/177Hf = 0.7325 and 173Yb/171Yb = 1.13268539 using an exponential correction for mass bias. Interference of 176Yb on 176Hf was corrected by measuring the interference-free 173Yb isotope and using 176Yb/173Yb = 0.7963939 to calculate 176Yb/177Hf. Similarly, the relatively minor interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Lu isotope and using the recommended 176Lu/175Lu = 0.0265640 to calculate 176Lu/177Hf. We used the mass bias of Yb (β Yb ) to calculate the mass fractionation of Lu because of their similar physicochemical properties. The lack of correlation between 176Hf/177Hf and 176Yb/177Hf (Fig. S6) is an indicator that the corrections imposed to the 176Hf/177Hf ratio result in accurate and precise data. Off-line selection and integration of analytic signals and mass bias calibrations were performed following those of Liu et al.35.

The Lu–Hf and Sm–Nd isotope analyses for the whole rock and mineral separates were conducted on a ThermoElectron Neptune™ multi-collector (MC–) ICP-MS in the GeoAnalytical Laboratory at Washington State University. About 200 mg of clean, alteration free amphibole, pyroxene and plagioclase were hand-picked under a binocular microscope. Sample dissolution and chemical separations are described by Cheng et al.41,42 and the full data set is presented in Supplementary Tables. Epsilon Hf and Nd values were calculated using 176Hf/177Hf = 0.282785, 176Lu/177Hf = 0.0336, 143Nd/144Nd = 0.512630 and 147Sm/144Nd = 0.1960 for CHUR43.