DTD 5 ARTICLE IN PRESS Earth-Science Reviews xx (2005) xxx–xxx 1 www.elsevier.com/locate/earscirev 2 How the delamination and detachment of lower crust can influence 3 basaltic magmatism 4 Michele Lustrino 5 Dipartimento di Scienze della Terra, Universita` degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Rome, Italy 6 Istituto di Geologia Ambientale e Geoingegneria (IGAG)-CNR, P.le A. Moro, 5, 00185 Rome, Italy 7 Received 26 July 2004; accepted 31 March 2005 8 9 Abstract 10 The Earth’s lithosphere can focus basaltic magmatism along pre-existing weakness zones or discontinuities. However, apart 11 from the influence on the geochemistry of magmas emplaced in subduction tectonic settings (mantle wedge metasomatism 12 related to dehydration of the subducting plates) the role of lithosphere as a magma source for intra-plate (both oceanic and 13 continental), continental margin, and mid-ocean ridge magmatism is not yet fully understood. In many cases intra-plate 14 magmatism has been explained with the existence of deep thermal anomalies (mantle plumes) whose origin has been placed 15 near the upper–lower mantle transition zone (660 km discontinuity) or even deeper, near the mantle–core boundary (~2900 16 km). Also in many continental flood basalt provinces (mostly initiated at craton margins) an active role for mantle plumes has 17 been invoked to explain the high melt productivity. In these cases, no active role for melt production has been attributed to the 18 lithospheric mantle. Potential contaminations of asthenospheric or even deeper mantle melts are often considered the only 19 influence of the lithosphere (both crust and mantle) in basalt petrogenesis. In other cases, an active role of the lithospheric 20 mantle has been proposed: the thermal anomalies related to the presence of mantle plumes would trigger partial melting in the 21 lithospheric mantle. At present there is no unequivocal geochemical tracer that reflects the relative role of lithosphere and upper/ 22 lower mantle as magma sources. In this paper another role of the lithosphere is proposed. 23 The new model presented here is based on the role of lower crustal and lithospheric mantle recycling by delamination and 24 detachment. This process can explain at least some geochemical peculiarities of basaltic rocks found in large igneous provinces, 25 as well as in small volume igneous activities, as well as in mid-ocean ridge basalts. Metamorphic reactions occurring in the 26 lower continental crust as a consequence of continent–continent collision lead to a density increase (up to 3.8 g/cm3) with the 27 appearance of garnet in the metamorphic assemblage (basaltYamphiboliteYgarnet clinopyroxenite/eclogite) leading to 28 gravitative instability of the overthickened lithospheric keel (lower crust+lithospheric mantle). This may detach from the 29 uppermost lithosphere and sink into the upper mantle. Accordingly, metasomatic reactions between SiO2-rich lower crust partial 30 melts and the uprising asthenospheric mantle (replacing the volume formerly occupied by the sunken lithospheric mantle and 31 the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures. Such metasomatized mantle 32 volumes may remain untapped also for several Ma before being reactivated by geological processes. Partial melts of such 33 sources wouldUNCORRECTED bear strong lower crustal signatures giving rise to Enriched Mantle PROOF type 1 (EMI)-like basaltic magmatism. E-mail address: [email protected]. 0012-8252/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.earscirev.2005.03.004 EARTH-01374; No of Pages 18 ARTICLE IN PRESS 2 M. Lustrino / Earth-Science Reviews xx (2005) xxx–xxx 34 Basaltic magmatism with such a geochemical signature is relatively scarce but in some cases (e.g., Indian Ocean) it can be a 35 geographically widespread and long-lasting phenomenon. 36 D 2005 Published by Elsevier B.V. 37 Keywords: Delamination; Detachment; Lithosphere; Mantle plume; Petrology; Lower crust; Eclogite; Pyroxenite 38 39 1. Introduction 2. Geochemical expression of mantle convection 79 40 Korenaga (2004) recently proposed an alternative 2.1. Are mantle plumes necessary? 80 41 to the mantle plume theory, that can explain 42 voluminous magmatic activity associated with con- Since at least the 1960s, many studies have 81 43 tinental break-up. His model suggests that upper invoked strong thermal anomalies as the main cause 82 44 mantle heterogeneities are the most important factors of large volume igneous activity in both continental 83 45 for the high melt productivity of the Continental and oceanic settings (e.g., Wilson, 1963; Morgan, 84 46 Flood Basalt (CFB) provinces around the Atlantic 1971; Condie, 2001; Marzoli et al., 1999; Kamo et 85 47 Ocean. However, according to this model, transport al., 2003; Thompson et al., 2003; Ewart et al., 86 48 of material from the lower mantle still plays an 2004). These plume-like thermal anomalies were 87 49 important role. In particular, the key control respon- considered to be near-cylindrical in shape except for 88 50 sible for the Atlantic CFB provinces would be the a huge, mushroom-shaped, head, with mean diam- 89 51 subducted crustal material (the uppermost portion of eters on the order of several hundred to thousand 90 52 subducting slabs) stored at the upper–lower mantle kilometers (Campbell and Griffiths, 1990; Farnetani 91 53 transition zone at ~ 660 km in depth. The model et al., 2002). Mantle tomography gives contrasting 92 54 proposed by Korenaga (2004) can be modified and results about the existence of the mantle plumes, 93 55 exported in many other cases where CFB activity either indicating their existence (e.g., Montelli et al., 94 56 develops. Many other recent papers also invoke 2004) being ambiguous (e.g., Ritsema and Allen, 95 57 chemical heterogeneity in the shallow mantle to 2003) or precluding the possibility of deep-rooted 96 58 explain features of ocean and continent magmatism origin for these anomalies (e.g., Anderson et al., 97 59 (e.g., Meibom and Anderson, 2003). In this paper I 1992). The presence of mantle plumes has also 98 60 emphasize an alternative model for the continental been invoked to explain small volume igneous 99 61 break-up and the associated magmatism that does not activity both in continental and oceanic settings 100 62 require the existence of mantle plumes with deep (e.g., Hoernle et al., 1995; Chauvel et al., 1997; 101 63 roots in the lower mantle. The model presented here Wilson and Patterson, 2001; Hildenbrand et al., 102 64 involves delamination and detachment of lower crust 2004). The source regions of such thermal anoma- 103 65 to mantle depths and is not necessarily an alternative lies are considered to lie at the bottom of the lower 104 66 to the Korenaga’s (2004) model. Recycling of lower mantle (the core–mantle boundary; CMB), the 105 67 crust (coupled with lithospheric mantle) can explain upper–lower mantle transition zone (the 660 km 106 68 several geochemical peculiarities relatively common discontinuity) or somewhere at mid-mantle depths 107 69 to low-volume intra-plate igneous rocks (ocean (below the 660 km and above the 2900 km 108 70 island basalts and intra-continental rocks), oceanic discontinuities; i.e., Cserepes and Yuen, 2000; Zhao, 109 71 and continental flood basalts and mid ocean ridge 2001; Courtillot et al., 2003; Ritsema and Allen, 110 72 basalts (MORB). 2003; Montelli et al., 2004). Since the late 1990s, 111 73 The first part of this paper highlights paradoxes the classical plume model has been the subject of 112 74 and inconsistenciesUNCORRECTED of the classic mantle plume strong criticism. PROOF Many contradictory aspects have 113 75 model, the second part reviews geochemical and been reviewed, among others, by Sheth (1999), 114 76 geophysical evidence for lower crust/lithospheric Smith and Lewis (1999) and Anderson (2002; 115 77 mantle delamination, while a more detailed model is reviewed at the web site http://www.mantleplumes. 116 78 developed in the final section. org). The most important problems confronting 117 ARTICLE IN PRESS M. Lustrino / Earth-Science Reviews xx (2005) xxx–xxx 3 118 mantle plume models can be summarized as portions of the lithosphere in channeling the CFB 166 119 follows: feeding systems. 167 120 e) There is a paradox between the bpin-pointQ 168 121 a) Experimental studies (e.g., Cordery et al., 1997; expression of bplumeQ thermal anomaly and the 169 122 Yaxley, 2000) cannot explain the high melt planar development of continental rifts. This 170 123 productivity of Large Igneous Provinces (LIPs) consideration is closely connected to (d) above, 171 124 without recourse to the existence of a relatively thereby reinforcing the evidence for strong litho- 172 125 low-temperature melting material resembling crus- spheric control on the localization of LIPs and the 173 126 tal rocks stored at various depths in the mantle. development of continental rifting (e.g., Vauchez et 174 127 This means that the origin itself of the huge al., 1997; Tommasi and Vauchez, 2001). 175 128 volumes of magmas produced in LIPs may f) None of the known hotspots meets all five criteria 176 129 ultimately derive not necessarily (or, better, not for detecting ultra-deep origins cited by Courtillot 177 130 entirely) from anomalously hot geothermal gra- et al. (2003): (1) monotonic age progression of 178 131 dients but, rather, from anomalous fertile sources linear volcanic chain; (2) flood basalts at the origin 179 132 (fertility being consequence of the presence of Ca– of this track; (3) a large buoyancy flux; (4) high 180 133 Al–Fe-rich eclogitic slices mixed in a peridotitic 3He/4He and 21Ne/22Ne isotopic ratios, and (5) low 181 134 medium).