4. Década de aplicación de la Ley de Violencia de
4.3. Niños, Adolescentes y Jóvenes afectados por la Violencia de
The early rifting stage, from 5 to 3.0 Ma, was a period of maximum volcanic
' .
activity in Fiji, coinciding with well-developed spreading in the NFB (eg. Auzende et al., 1995) and maximum rotation of the Fiji Platform (eg. Begg and Gray, 2002). In contrast to the andesite-dominated volcanism discussed above (section 4.4.2),
volcanism during the early rifting stage was predominantly mafic and shoshonitic. Shoshonitic volcanism during this period was also accompanied by the eruption of
subordinate volumes of calc-alkaline and arc-tholeiite rocks (Gill and Whelan, 1989a). Criteria for defining "rock series" ego shoshonitic, calc-alkaline and
tholeiitic-series follow that of Gill and Whelan (1989a) and are outlined in Table 4.4.3A (see also Section 2.1). Following the classification scheme of Peccerillo and Taylor (1976), the three rock series are shoshonitic, high-K and medium-K respectively (Gill and Whelan, 1989a).
The main centres of Pliocene Fijian volcanism are shown in Figure 4.4.3A. The structural control of volcanism during the early rifting stage is clear in the aligrunent of the majority of Pliocene volcanic centres along three broad ENE and NNW trending lineaments; the North Viti Levu Lineament, Lomaiviti Lineament and the Vatulele-Beqa Lineament (Figure 4.4.3A). As noted by a number of workers (eg. Colley and Hindle, 1984; Broc
�
er and Holmes, 1985; Gill and Whelan, 1 989a), such trends are parallel and/or perpendicular to major transform and ridge features that are presently active (eg. HFZ, FFZ, and the Western Propagating Ridge of the NFB).Shoshonitic rocks erupted from 1 1 main volcanic centres (Figure 4.4.3A). Shoshonitic centres associated with the Viti Levu Lineament include: Sabeto, Koroyanitu and Tavua volcanoes; Lomaiviti Lineament; Vatu-i-Cake, Wakaya, Gau and Moalit Islands; Vatulele-Beqa Lineament; Vatulele and Yanuca Islands. In addition, the Astrolabe Islands of Kadavu form an additional NNE aligned shoshonitic centre that does not appear to conform to the other main lineaments, and notably, occurs outside the Fiji Platform, as defined by the 2000m bathymetric contour.
Gill
and Whelan (1989a) categorised the island of Moala as a calc-alkaline centre. However, radiometric dating by Rodda and Kroenke (1984) presented an age of 4.1' .
± 0.2 Ma on a shoshonitic flow near Vunuku village. This occurrence is supported
by fieldwork by the author in June of 2001, who noted extensive augite-olivine (absarokite) flows (eg. sample MO-4 Table 5.2C) and lava breccias on eastern Moala, that are conformably overlain by peridotite-nodule-bearing alkali-basalts.
IOOkm
KADAVU GROUP
.& Pliocene shoshonitic volcanic centres
.& Pliocene calc-alkaline volcanic centres
I 800
.& Pliocene tholeiitic volcanic centres (Korobasaga Volcanic Group, Lau)
Late Miocene - Early Pliocene Natewa and Monkey Face Volcanic Groups
Figure 4.4.3A Distribution of early rifting stage volcanic centres 5-3 Ma, after Gill and Whelan, (J989a). Shoshonitic centres: SB - Sabeto; NU - Nausori; KO - Koroyanitu; TV - Tavua; RR - Rakiraki; VT -Vatu-i-Cake; WK - Wakaya; GU - Gau; MO - Moala; AS -Astrolabe Group; YN -
Yanuca; VU - Vatulele. Calc-alkaline Centres: NC - Nacilau; NM - Namosau; VA - Vatia; RR - Rakiraki; TO - Tova; OV - Ovalau; NM - Namenalala; MN - Mokagai; BT - Batiki; MO - Moala; TT - Totoya; MT - Matuku. Lomaiviti Tholeiitic Centres: YD - Yadua; NR - Narai. See text for detail.
Table 4.4.3A. Characteristics of basalts
« 53
wt% Si02) of the early rifting stage.From Gill and Whelan (1989a).
Shoshonitic Calc-alkaline Tholeiitic
KaO @ 50% SiOa >1 .8% 1 .0-1 .8% <1 .0% NaaOlKaO 0.5-1 .4 1 .4-2.0 3-5
Norm % 1 -1 0 ne <2 ne, <5 hy > 1 0 hy
Rb/Zr 0.6-1 .2 0.35-0.50 0.20-0.30
Table 4.4.3B K-Ar age data for Fijian shoshonites. From Whelan et al. (1985).
Island I Volcanlc centre sample number # age (Ma ± 2 s.d)
V1U Lew Uneament
Sabeto W8 4.71 ± 0.05 Sabato Wl0 3.97 ± 0.09 Sabato W18· 3.94 ± 0.04 Sabato 4.29 ± 0.10 Sabato W27 5.21 ± 0.07 Koroyanltu Y695 4.78 ± 0.20 Koroyanltu Y232 4.96 ± 0.30 Tavua W37* 3.86 ± 0.06 Tavua 4.04 ± 0.06 Tavua W61a 2.56 ± 0.78 Tavua W930 4.76 ± 0.06 Tavua W930C 4.03 ± 0. 1 3 Tavua W930BT 4.43 ± 0.36 Tavua W68 3.02 ± 0.04 Tavua W67 3.51 ± 0.07 Tavua W66 2.48 ± 0.48 Tavua 472 3.93 ± 0.48 Tavua S16 5.05 ± 0. 1 7 Tavua VL26c 5.1 8 ± 0.10 LomalviU Uneament Wakaya 223a 3.54 ± 0.13 Vatu-l-Cake 233· 3.84 ± 0.05 Vatu-l-Cake 3.83 ± 0.07 Gau 280 3.40 ± 0.10 Gau 284 3.26 ± 0.04 Gau 285 3.03 ± 0.06 Vatulele-Beqa Uneament Vatulele 301 4.74 ± 0.30 VatuJele 310 4.72 ± O.19 Yanues 31 1 4.00 ± 0.19 Ugaga 317 4.21 ± 0.06 Beqa 320 3.07 ± 0.08 Astrolabe Group Namara 328a 3.39 ± 0.04 Dravunl 330 3.40 ± 0.10
en
�
... c: Q) (,,) (,,) E c: 0 ..c: en 0 ..c: en Q) >t;
m 8 6 4 2 o 5.4 5.2 5 4.8 4.6 4.4 4.2 4Age
Ma 3.8 3.6 3.4 3.2Figure 4.4.3B. Histogram illustrating number of active shoshonitic centres in Fiji during the Pliocene. Data from Whelan et al. (1985).
Moala absarokites have K20 := 1 .9 wt%, K20INa20 = 0.86 and 0.64 wt% Ti02 at 48
wt% Si02 and 13.8 wt% MgO and clearly qualify as shoshonitic following the
criteria of Morrison (1980), Gill and Whelan, (1989a) ego Table 4.4.3A, or that of Peccerillo and Taylor (1976). It would appear that the Moala volcanic centre erupted a continuum, or progression of compositions, from medium-K calc-alkaline to shoshonitic, culminating in alkali-basalt flows (see. 4.4.4 below).
The K-Ar age data of Whelan et al. (1985), (Table 4.4.3B, Figure 4.4.3B) suggest that shoshonitic volcanism was initiated on the Viti Levu Lineament at the Sabeto, Koroyanitu and Tavua centres by �5.2 Ma, or possibly as early as 5.5 Ma (McDougall, 1963), with continuing activity until � 2.5 Ma. By �4.7-4.2 Ma shoshonitic volcanism had spread to the Vatulele-Beqa Lineament, with subsequent eruptions occurring along the Lomaiviti Lineament (�4.1-3.0 Ma) and at the Astrolabe volcanic centre (�3.4 Ma). The general geology and geochemistry of Fijian shoshonitic suites will be d,iscussed in the following chapter.
During the same time period, calc-alkaline magmas (as defined in Table 1 , Gill and Whelan, 1989a) erupted from adjacent volcanoes (Figure 4.4.3). Along the Lomaiviti Lineament, calc-alkaline volcanoes occur approximately midway between shoshonitic centres (eg. Mokagai and Batiki). Gill and Whelan (1989a) suggested that in all three lineaments the westernmost volcanoes are shoshonitic ego Sabeto (Viti Levu), Vatulele (Vatulele-Beqa) and Vatu-i-Cake (Lomaiviti), whereas the eastenimost volcanoes are calc-alkaline ego Rakiraki, Beqa, Totoya and Matuku. However, from Figure 4.4.3A this progression is not obvious, and additionally, several calc-alkaline volcanoes (eg. Ovalau, Namena and Tova) do not appear to be associated with any of the main lineaments. The majority of calc-alkaline centres are basalts and basaltic andesites, with andesities occurring only at Namosua and Vitia in northern Viti Levu. Basalts and basaltic andesites contain augite, plagioclase and magnetite phenocrysts ± olivine, with hornblende commonly
present in the more evolved andesites; orthopyroxene is absent (Gill and Whelan, 1989a). Gill and Whelan (1989a) suggested that overall, shoshonitic and calc- alkaline suites are much alike, forming a continuum of increasing alkali and LILE enrichment in the sequence medium-K calc-alkaline to high-K calc-alkaline to
"
shoshonitic, with calc-alkaline suites generally more silicic and lacking biotite. Basalts of both magma series show approximately the same levels of LREE, Zr and 878r/868r enrichment (CeN = 1 .0, ZrN < 4, 878r/868r = 0.7036-43), which characterised the calc-alkaline, andesite dominated suites of the preceding mature arc stage
(Gill,
1987).Pliocene low-to medium-K tholeiities (Fig 4.4.3A) occur on Vaunua Levu (Natewa, Uluikamali and Monkey Face Volcanic Groups) and in the Lau Islands (Korobasaga Volcanic Group). These rocks are more mafic than the tholeiities of the preceding magmatic stages and consist predominantly of basalt and picrite with minor andesite. Mineralogically, they differ from the coeval calc-alkaline suites by containing more plagioclase and less magnetite in mafic basalts and phenocrysts of hornblende in addition to orthopyroxene in evolved andesitic rocks (Gill and Whelan, 1989a and references therein). Both Vauna Levu and Lau tholeiitic suites are broadly similar and have typical arc-like signatures (eg. BalLa>30), arc tholeiitic K20 concentrations (0.4-1.5 wt % K20), flatter REE patterns (CeN - 0.6) and lower Zr enrichment (ZrN <2.5) than contemporaneous shoshonitic and calc alkaline suites (Gill and Whelan, 1989a). However, Korobasaga Volcanic Group of the Lau islands has notably less radiogenic Sr (0.7033-35) than the tholeiities of Vanua levu (0.7036-41) (Gill and Whelan, 1989a).