IV. RESULTADOS Y DISCUSIONES
4.3. Análisis de Tarwi ( Lupinus mutabilis)
Figs.5.6a-c are ternary tectonic discrimination diagrams utilising immobile trace elements and major elements. On the Ti-Zr-Y diagram (Pearce & Cann, 1973), the South Bacan Formation rocks plot in the overlap fields of OFB and CAB; whereas on the Nb-Zr-Y diagram (Meschede, 1986) they plot in the fields o f N-MORB & VAB and VAB & WPT. The TiO;- M nO-PjOj diagram (after Mullen, 1983) shows them plotting in the CAB field.
Fig.5.6d shows the MORB normalised diagram o f Pearce (1982), where the South Bacan Formation rocks display the characteristics of arc rocks (humped LIL; Nb depletion; slightly humped Nb-Zr; fairly flat Zr-Sc, all below MORB values; and strong Cr depletion). Fig.5.6e is a chondrite normalised diagram (after Sun, 1980), showing the South Bacan Formation rocks having a pronounced Nb depletion; flat La-P distribution except K and Sr; flat Zr-Y with a slight Ti depletion; and a general decrease o f normalised elemental abundance with increasing
T h e S o u th B a ca n F o im a tio n Ti/100 LKT: A and B OFB: B CAB B a n d C W P 8 D Bo Fig.5.6b A : P-MORB
B: N-MORB & VAB C: VAB & W PT D: WPB Oo Z r/4, Flg,5.6c A; OIT B: OIA C;CAB D:IAT E: MORB M nO -Il 100; F Ig. 5.6d CQ 0.0 1'
C
BM282 1000,
Fig.5.6e 100 B M 1 8 8 - * - B M 2 5 0 - * - BM28BFigure 5.6a-c. Ternary tectonic discrim ination diagram s applied to the South Bacan Formation. Figure 5.6d-e. Spider diagrams for the South Bacan Formation volcanic rocks.
T h e S o u th B acan F orm ation
incompatibility, typical of arc-related volcanic rocks.
Figs.5.7a-e are covariation diagrams (Cr against Y and Ce/Sr; Zr against Zr/Y and Ti; Ti/Y against Nb/Y) used to decipher tectonic setting. In all diagrams, the South Bacan Formation rocks plot mostly in the VAB field.
5.6.4 Im plications
Trace element contents and ratios and tectonic discrimination diagrams all point to a volcanic arc origin for the South Bacan Formation. Com parison with older units reveals that the South Bacan Formation is similar to both the Saleh Com plex and the Bacan Formation. The resemblance to the Bacan Formation is particularly striking. Despite this resemblance, K-Ar and micropalaeontological ages from both formations are different. The relationship with the Saleh Complex is less clear. Comparison with the Nusa Babi M onzodiorite will be discussed in Chapter Nine.
5.7 A G E D E T E R M IN A T IO N
Two mudstone samples (BM207 and BM 218) have been dated using foram inifera (F.T. Banner, pers.comm, 1992) and indicate an Early M iocene (N4, Aquitanian-Langhian) age. Characteristic
faunas are: Globigerinoides sp(p), Globigerina sp(p), Dentoglobigerina cf altispira,
IParagloborotalia and Tenuitellids.
Two samples have been dated using the K -A r technique and they yield: 20.8 ± 2.0 Ma (BM250: float) and 7.5 ± 1.1 Ma (BM188) ages. The -21 Ma age is consistent with the biostratigraphic age, while the 7.5 Ma age is interpreted to be a reset ages related to the activities of the Halmahera arc. The older age is not affected by thermal events because o f the fresher nature o f the sample and the fact that the material analysed is hornblende separate, with a higher closure temperature than the whole rock. Figs.5.8a,b are isochron diagram s showing the age relationship between the South
Bacan Formation and the similar aged N usa Babi Monzodiorite {Chapter Nine).
T able 5 .3 Sum m ary o f the K -A r results from the S o u th B acan Form ation.
S am p le M aterial Grain siz e (p) %K ( l a error) W t for A r (g) ^ A r* (n l/g , l a error) * A ^ m (% ) A g e (M a. 2 a error) B M 2 5 0 hb 1 2 5 -2 5 0 0 .2 1 8 ± 1.00% 1.0 2 7 5 0 .1 7 6 8 ± 3.39% 7 3 .3 4 2 0 .8 ± 2 .0 B M 1 8 8 wr 2 5 0 -5 0 0 1 .2 8 9 ± 2 .27% 1 .0 2 3 6 0 .3 7 6 7 ± 7.13% 8 7 .3 8 7 .5 ± 1.1 1 6 2
Fig.5.7a
ù 10
100
Y ppm
T he South Bacan Form ation lOOCb lOCt Fig.5.7b 1000D Fig.5.7d
I
VAB i- 1000 1000 100 Zr ppm 1000 Flg.5.7c VAB M ORB 0.1 Nb/Y Rg.5.7e M ORB I I ' T I I 100 Zr ppm 1000Figure 5.7a-e. Trace elem ent covariations used to decipher the tectonic setting of the South Bacan Formation.
The South Bacan Formation
5.8 STRUCTURE
Small scale normal faults forming horst and graben structures were observed within this formation (Plate 5.1c). Differences in the six bedding attitudes from S. W ayatim (-160/40) with the four from S. Bibinoi (-310/45) suggest that the formation has been deformed. There are pervasive joints with directions, measured at different areas, o f 000 & 065; 030 & 100; 078 & 162; and 105 & 190. The interjoint angles are 60-90°, indicating conjugate sets. The presence o f large intrusions in S. Bibinoi area may be a cause of localised deformation, causing differences in bedding attitudes and joint directions in different areas.
5.9 SYNTHESIS AND TECTONIC SIGNIFICANCE
The South Bacan Formation consists of interbedded volcanic and volcaniclastic rocks erupted in an arc-related setting. This is supported by the whole rock geochemistry which clearly indicates a volcanic arc origin for the South Bacan Formation. Facies analyses o f sedimentary rocks suggest deposition by intermediate turbidity currents which dissipated their energies as they travelled down slope. All the samples examined have been affected by low temperature prehnite-pumpellyite facies metamorphism. The lack o f fabric development in the formation suggests a regional, static metamorphism, due to burial (~240-330°C, - 2 kb). Locally this formation shows the effects o f hydrothermal alteration.
Isotopic and micropalaeontological age analyses yielded Late Oligocene-Early Miocene ages, with evidence o f a thermal overprint related to the activities of the Halmahera arc. These ages are
similar to the age o f the Nusa Babi M onzodiorite {Chapter Nine).
The most striking feature o f this formation is its lithological and chemical similarities to the Bacan Formation and before dating they were assumed to be the same age. There are two possibilities to account for the age differences between the Bacan and South Bacan Formations: [ 1 ] the two formations are part o f a single formation spanning from Late Eocene to Early Miocene; the age differences may reflect either an older sequence at north Bacan or be a function o f incomplete sampling and [2] these two are genuinely different formations, hence the separate age and spatial distribution. This resemblance probably reflects the fact that two formations with similar lithological character and tectonic setting have been metamorphosed and deformed under similar conditions at the same time.
The continuous arc scenario is favoured here because o f the following reasons: [ 1 ] both formations post-date the Middle Eocene regional unconformity; [2] both formations are covered by the Middle
The South Bacan Formation
20 Ma ISOCHRON
450.0 400.0-I
? 350.0-I
300.0- (40 Ar/36Ar)i = 294.67 Age = 20.69 Ma 250.0-1---O.OOE+00 4.00E+04 8.00E+04
2.00E+04 6.00E+04 40K/36Ar
ISOCHRON (20Ma)
Age = 19.55 Ma
O.OOE+00 2.00E+00 4.00E+00 1.00E+00 3.00E+00
K(%)
Figure 5.8a-b. Isochron diagrams for the South Bacan Formation. Data includes the sim ilat aged Nusa Babi Monzodiorite.
The South Bacan Formation
Miocene Limestone (Ruta Formation), which throughout the southern Philippine Sea Plate region signifies post collisional formation, and [3] metamorphism and deformation on both formations must have taken place before the Middle Miocene, as the Ruta Formation is not affected by similar style metamorphism and deformation. It is, therefore, more logical to group the Bacan and South Bacan Formations together between the Middle Eocene (creation of Philippine Sea Plate and rapid northward subduction o f Australia) and Early Miocene (collision o f Australia with Philippine Sea Plate) regional unconformities. This suggests that these formations are related to subduction of Australia under Philippine Sea Plate. The missing Oligocene sequence in Bacan may be explained by movement of active eruptive centres and the Oligocene Tawali Formation o f Kasiruta may be
part o f this arc sequence {cf. Section 8.5.4).
Although this explanation is plausible, there are several arguments against it, particularly the difference in the Ethology, metamorphic character (e.g. the lower grade o f metamorphism suffered by the Tawali Formation relative to the underlying and overlying formations) and trace element differences between the Tawali and the Bacan/South Bacan Formations. The implication o f these on tectonic setting will be discussed in more detail in Chapter Eleven.
CHAPTER SIX
THE TAWALI FORMATION
6.1 INTRODUCTION 6.2 SYNONYMY
6.3 AERIAL PHOTOGRAPHY 6.4 TYPE AREAS
6.5 THE JOJOK MEMBER 6.5.1 Lithofacies
6.5.2 Petrography and Mineral Chemistry
6.5.2.1 Petrography Ô.5.2.2 Mineral Chemistry 6.5.2.3 Implications
6.5.3 Whole Rock Chemistry
6.5.3.1 M ajor Elements 6.5.3.2 Trace Elements
6.5.3.3 Tectonic Discrimination Diagrams 6.5.3.4 Implications 6.5.4 Age Determination 6.6 MARIKAPAL MEMBER 6.6.1 Lithofacies 6.6.2 Petrography 6.6.3 Age Determination
6.6.4 Depositional Environment and M echanism
6.7 STR U C TU R E
6.8 SYNTHESIS AND TECTONIC SIGNIFICANCE
CHAPTER SIX
THE TAWALI FORMATION
6.1 INTRODUCTION
There are two members o f the Tawali Formation: the Jojok M ember and the Marikapal Member. The term Jojok Member is used for pillow basalts of arc-related character with associated hemipelagic rocks. The term Marikapal Member is applied to thick volcaniclastic turbidites associated with debris flows and red mudstones which typically overlie the lavas. These two members are easily distinguished in the field, but cannot be separated on aerial photographs. Since the mode of mapping in this region is principally by aerial photographs, the two members are considered as part of one formation. The regional distribution of this formation is shown in Fig.6.1.
This chapter is concerned with lithological and petrographical description o f both the Jojok and Marikapal Members, mineral and whole rock chemistry o f the Jojok Member, age determination and structural evidence from the formation.
6.2 SYNONYMY
The Tawali Formation has not been previously described. Yasin (1980) mapped it as part o f his Oligocene Bacan Formation, and correlated it with the Bacan Formation in Bacan, Halmahera and
Morotai. His Bacan Formation, however, includes rocks of different lithologies and ages {see
Section 4.2). Hakim & Hall (1991) and Hall et al. (1991) referred to this formation as the Kasiruta
Formation and correlate it with the Kahatola Formation of Halmahera and the Mayalabit Formation o f Waigeo.
6.3 AERIAL PHOTOGRAPHY
Aerial photographic examination reveals that this formation is characterised by rugged, continuous ridges o f similar height, which appear to be part o f dissected plateaux. W hen cut by rivers, the valleys formed are very steep. The drainage pattern is similar to those o f the Kaputusan, South Bacan and Bacan Formations.