Principio de calidad de los datos

Prior to addressing subduction-related geochemical enrichment processes, it is beneficial to elucidate the nature of the unmodified (ie. pre-subduction) mantle sources beneath the Fijian region.

There is a general consensus among studies of arc magmatism that HFSE and HREE (eg. Ta, Nb, Ti, Zr,

Yb

and Y) are not significantly mobile in fluids, or enriched in pelagic sediments, and are therefore suitable elements from which to constrain the background composition of the mantle wedge independent of subduction-related enrichment processes (Pearce and Parkinson, 1 993; Davidson, 1 996).

This is best illustrated by the MORB-array (shaded field) on Figure 1 0. 1 .2. This array reflects both variations in the degree of melting 'in the formation of MORB and the composition of the mantle source. Zr, Nb and

Yb

are elements that behave incompatibly during partial melting of mantle peridotite and have different Kd values (Sun and McDonough, 1 989). Consequently ratios of these elements ego ZrlYb and NbIYb (Figure 1 0. 1 .2) will vary systematically in the melt, with the degree of partial melting (ie. NbIYb and ZrlYb will increase with decreasing degree

.

of partial melting), and the mantle residues after varying degrees of melt extraction will therefore have different values (Pearce and Parkinson, 1 993). Furthermore, metasomatism by low-degree silicate melts (eg. McKenzie, 1 989) may also alter the ratios of these incompatible elements in the mantle residue and may lead to an increase of both NbIYb and ZrlYb. However, because these elements have similar

Ko values (similar incompatibility) in mantle peridotite, both partial melting (melt extraction) and metasomatism by low degree silicate melts, can effectively produce th�. same array and accordingly. Consequently, it is not possible to discriminate between each process (ie. partial melt extraction versus metasomatism by small­ degree silicate melts).

The ZrIYb versus NbIYb plot (Figure 1 0. 1 .2A) therefore provides a proxy indication of the relative depletion or enriclunent of the mantle source for Fijian shoshonites. Tavua samples (inclusions and rocks) span a wide range of ZrNb­ NbNb values, lying wholly within the MORB array, although the majority of samples display lower (more depleted) NbNb values than the average N-MORB (Sun and McDonough, 1 989). Tauva suite samples lie within the field of Tonga­ Kermadec arc rocks and overlap with the 10w-NbNb end of the Tabar-Feni (Papua New Guinea) shoshonitic field (Figure 1 0. 1 .2A).

Vatu samples display slightly more enriched ZrNb-NbNb values relative to Tavua samples and average N-MORB (Sun and McDonough, 1 989), overlapping with the least depleted end of the Tavua array and corresponding closely to the fields of calc­ alkaline rocks from the Vanuatu arc and Tabar-Feni shoshonitic rocks (Figure 1O. 1 .2A).

Astrolabe Group samples are significantly more enriched relative to Tavua and Vatu shoshonitic suites, forming a wide array on Figure 1 O. 1 .2A overlapping with NbNb values of average E-MORB (Sun and McDonough, 1 989) and shoshonites from the Northern Seamount Province (NSP) of the Mariana arc. Rocks from the Astrolabe Group display large variations in absolute Nb abundance and ZrlNb (Table 5.2C). In Section 5.2.3 it was noted that shoshonites (sensu stricto) from the westernmost Astrolab� islands (Namara, Yanuyanu-i-Loma and Yanuyanu-i-Sau, (see Figure 5.1 .3) are characterised by higher Nb concentrations and lower ZrlNb relative to shoshonites (sensu stricto) from the eastern Astrolabe islands (Yaukuvelevu, Yaukuvelailai and Buliya). For instance, comparison between shoshonite samples AV1 77 (Yaukuvelailai) and AVl 90 (Yanuyanu-i-Loma) (Table 5.2C) with 4.74 and 4.47 wt% MgO respectively, indicates an almost 3-fold difference in absolute Nb concentration at similar MgO, with ZrlNb values of 1 5 . 1 and 9.3 respectively (see Section 5.2.3). Notably, a similar variation is observed in primitive Astrolabe Group melt inclusions with ZrlNb ranging from 22.9-8.7 (Figure 9.3 .2M). These variations in ZrlNb and ZrNb-NbNb are interpreted to reflect an enriched mantle source component involved in the petrogenesis of Astrolabe Group shoshonitic magmas. It is worth noting that similarly low ZrlNb values (-9) were reported by Gill and

Whelan (1989a) for shoshonites (sensu stricto) from the nearby Vatulele shoshonitic centre (see Figure 4.4.3, and Section 5.2).

The HFSE and HREE systematics of the three Fijian shoshonitic suites indicate significant variations with regard to the composition of the sub-Fijian mantle source independent of subduction-related enrichment. Notably, Tavua and Vatu suites are situated in what is currently the northern part of the old arc-crust of the Fijian Platform.

In

contrast, the Astrolabe Group islands and Vatulele are situated south of the Fiji Platform. However, it remains unclear as to why mantle sources, currently south of the main Fiji Platform, should be enriched relative to mantle beneath what is currently the northern part of the Fiji Platform (ie. beneath Tavua and Vatu). Astrolabe Group and Vatu suites yield similar ages (-3 .4 and 3.8Ma respectively, Whelan et aI., 1 985, Section 4.4.3) and are -1 .5Ma younger than Tavua, while Vatulele is essentially the same age as Tavua (4.7Ma, Whelan et aI., 1 985, Section 4.4.3). However, Vatu shares similarly high ZrlNb as Tavua (Figure 9.3 .2M), suggesting that the variations in ZrlNb among the F

!

jian shoshonitic suites can not be strictly related in a temporal sense. Consequently, the more enriched southern most shoshonitic centres (Vatulele-Astrolabe Group) are interpreted to reflect involvement of a fertile mantle source component similar to average E-MORB (Figure 1 0.2. 1 A).

In

general it would appear that the variations in relative mantle enrichment and depletion are spatially related, with northernmost shoshonitic suites (Tavu,? and Vatu) displaying a greater degree of mantle source depletion relative to Astrolabe Group and shoshonitic centres associated with the Vatulele-Beqa lineament in the south (Figure 4.4.3). hnportantly, these areas or domains of more depleted mantle beneath the northern part of the Fijian Platform (ie. beneath Tavua and Vatu) should be more sensitive to additions of incompatible elements from the subducted slab, relative to the more enriched mantle sources further south beneath the Astrolabe Group-Vatulele region.

10.1.3 Fijian shoshonitic magmas: an expression of subduction­

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