The Lithospheric Thickness Control on the Compositional Variation of Continental Intraplate Basalts: A Demonstration Using the Cenozoic Basalts and Clinopyroxene Megacrysts From Eastern China

Abstract

Studies on intraplate ocean island basalts have demonstrated the control of lithosphere thickness on the extent of melting and pressure of melt extraction (i.e., the lid effect). However, whether lithosphere thickness also controls the composition of within‐continent basalts remains unclear. Here, we test this hypothesis by studying the Cenozoic basalts contaitning clinopyroxene megacrysts from 10 localities throughout eastern continental China with a north‐south spatial coverage in excess of 2,500 km. Indeed, the geochemical parameters (e.g., abundances and ratios of major and trace elements) correlate well with the depth of the lithosphere‐asthenosphere boundary (LAB) calculated using the clinopyroxene barometry, showing significant lithospheric thickness control on basalt compositions. These observations offer further evidence for melt pooling (a melt rich layer) close beneath the LAB as a “stable magma reservoir” for crystallizing compositionally uniform clinopyroxene megacrysts to be carried by subsequent pulses of melt transport and eruption.

Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia

“…A fertile mantle source has higher SiO 2 , Al 2 O 3 , FeO T , and TiO 2 contents and lower MgO content than less-fertile mantle [48,49]. Melt SiO 2 and Al 2 O 3 contents increase and FeO and MgO contents decrease with decreasing melting pressure [37]. Previous studies have suggested that the mantle source of Pohnpei Island is relatively homogenous [10,13], and the variations of FeO T and TiO 2 with MgO (Figure 4b,d), and SiO 2 and Al 2 O 3 with FeO T (not shown) thus reflect the effect of melt extraction pressures rather than mantle composition.…”

“…Major-element data from previous studies are included for comparison [9,10,18]. Compositional trends [37] of melts derived from less to more fertile mantle (green arrows) and from high to low pressures of melt extraction (purple arrows) are shown in (a-d). [32], are included.…”

“…The calculated compositions of primary melts are shown in Table 2 and Figure 4. The modelled primary magma exhibits moderate chemical variations, with those of SiO 2 and Al 2 O 3 with MgO in Pohnpei primary magmas being affected by the mantle source and/or melt-extraction pressure (Figure 4a,c; [37]). A fertile mantle source has higher SiO 2 , Al 2 O 3 , FeO T , and TiO 2 contents and lower MgO content than less-fertile mantle [48,49].…”

“…Previous studies have indicated that the age and thickness of the oceanic and continental lithosphere exert an important influence on the compositional variation of intraplate OIB-type lavas [17,37,52,53,63,64]. For example, Niu et al [53] demonstrated that, on a global scale, the geochemical compositions of OIBs erupted on thick lithosphere are consistent with a low degree of partial melting under high pressure, while those erupted on thinner lithosphere exhibit a higher degree of partial melting, with melting ceasing at shallower depths.…”

“…For example, Niu et al [53] demonstrated that, on a global scale, the geochemical compositions of OIBs erupted on thick lithosphere are consistent with a low degree of partial melting under high pressure, while those erupted on thinner lithosphere exhibit a higher degree of partial melting, with melting ceasing at shallower depths. Sun et al [37] found that lithospheric thickness variations exert primary control on the geochemistry of intra-continental basalts. Zhang et al [13] have shown that Caroline Plateau alkaline lavas have higher SiO 2 and Al 2 O 3 contents and lower FeO T contents and Sm/Yb ratios than those of the Caroline Islands.…”