Petrographic Characterization of Extrusive Rocks along

Banahaw de Lucban, Banahaw Volcanic Complex, Quezon

Province

Jeremy Lemuel M. Ravina

MAPÚA INSTITUTE OF TECHNOLOGY

School of Civil, Environmental, and Geological Engineering

dbsenoro@mapua.edu.ph

(+63 2) 2475000 local 5109

June 2016

CEGE Journal

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Abstract

Banahaw Volcanic Complex (BVC), located on the easternmost part of Macolod

Corridor, is composed of the main cone referred to as Mt. Banahaw and the

parasitic cones namely Pinagdanglayan, Mt. San Cristobal and Banahaw de

Lucban. Historical eruptions were recorded for Mt. Banahaw only, which

provided important information not only on the nature of its eruption, but also on

its geologic characteristics. The complexity of this volcanic system is only little

understood given the lack of geological interest to determine its lithologic and

petrographic attributes. In contrast, very little is known for the parasitic cones. In

light of this dilemma, this paper is written with the aim to petrographically

characterize the rocks along Banahaw de Lucban. A fieldwork along the parasitic

cone was conducted to collect outcrop samples. The collected samples were

initially analyzed megascopically, then petrographically for detailed mineralogical

and textural information. The results of the petrographic analysis yields into

identification of four different rock types corresponding to four eruptive events.

Based from field occurrence, oxyhornblende andesite is the earliest eruptive

product among the four. However based on its intermediate composition, it is

attributed to the youngest eruption of Mt. Banahaw, rather that of Banahaw de

Lucban, whose eruptives are mafic in composition. The earliest eruptive of

Banahaw de Lucban is attributed to the intergranular pyroxene basalt, followed by

the overlying intersertal pyroxene basalt. The latest eruptive of Banahaw de

Lucban which is present on the peak is orthophyric pyroxene andesite. Based

from the similarities on the mineralogy of pyroxene basalts of Banahaw de

Lucban, the span of time between each eruptions of must have been short, and

probably be an episode of eruption from a single eruption event.

Keywords: Macolod Corridor, Banahaw Volcanic Complex, Banahaw de Lucban,

petrography, magmatic history.

Acronym

BVC Banahaw Volcanic Complex

AN Anorthite

1 Introduction

The Philippines, being an island arc, is marked by the presence of several volcanic belts

almost throughout its stretch. In Luzon, one of the volcanic belts is the Macolod corridor (Figure

1a and 1b) which is comprised of the following from west to east: (1) the Taal Volcano (2) the

monogenetic volcanoes and; (3) the Laguna de bay and Banahaw Volcanic Complex (BVC)1. Of

these volcanic systems, the BVC seemingly is the least studied, being hardly mentioned in past

literatures.

Petrographic studies on volcanic rocks are numerous, providing basic, yet crucial

information on the magmatic history of the rocks. In detail, petrography aids in determining with

greater accuracy the rock’s mineralogical composition, texture and order of crystallization. These

data, in turn, provides as basis for deducing the differentiation processes the rocks had undergone.

1.1 Background and Problem Motivation

The BVC, which lies in the jurisdiction of Laguna and Quezon provinces, is composed of

the main cone referred to as Mt Banahaw, and the parasitic cones namely (a) Pinagdanglayan; (b)

San Cristobal, (c) Banahaw de Lucban (BDL)2. A shown in Figure 2, BDL is located northeast of

Mt. Banahaw in the municipality of Lucban, Quezon Province and is approximately centered at

geographic coordinates of 14°4’ N and 121°30' E. Little is known for the parasitic cones of the

BVC, given the lack of geological interest to determine its lithologic and petrographic attributes

and how these relate to the BVC’s geological characteristics. It is in this view that this research is

proposed, looking into the petrographic characterization of the parasitic cones, with particular

emphasis on the Banahaw de Lucban.

Figure 1. Macolod Corridor

__________________________ 1 Forster, H., Oles, D., Rnittel, U., Defant, M.J. and Torres, R.C., (1990). “The Macolod

Corridor: A rift crossing the Philippine island arc”. In: J. Angelier (Editor), Geodynamic

Evolution of the Eastern Eurasian Margin. Tectonophysics, 183: 265-271. 2 PHIVOLCS (Philippine Institute of Volcanology and Seismology). Volcano List-Banahaw

Volcano. Retrieved August 27, 2015 from

http://www.phivolcs.dost.gov.ph/html/update_VMEP/Volcano/VolcanoList/banahaw.htm

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Figure 2. Geographic Location of Banahaw de Lucban

1.2 Overall Aim

Given the limited geologic information on the rocks comprising the Banahaw de Lucban,

this study aims to identify Banahaw de Lucban’s eruption history based on the rocks’ petrography

and field occurrence. Baseline information on the petrologic and petrographic characteristics of

Banahaw de Lucban will be provided by this study. This information is useful for future research

including geochemical characterization of the rocks. Insights on the cooling and crystallization

history of the rocks would also provide crucial information on Banahaw de Lucban’s past eruptive

events which in turn, are important in formulating models pertaining to the volcano’s future

magmatic activities.

1.3 Scope

The study is only limited on the characterization s and interpretation of the related

magmatic processes of extrusive rocks along Banahaw de Lucban. The study area of this research

lies in the northern portion of Banahaw de Lucban, particularly in the jurisdiction of Barrio Samil

in Lucban, Quezon and Barrio Taytay in Majayjay, Laguna. Additionally, accessibility and outcrop

occurrence of Banahaw de Lucban is limited to the northern portion due to thick forest cover of

the mountain.

1.4 Concrete and Verifiable Goals

In line with the main objective, three specific objectives are set:

1. Characterize the rocks based on their mineralogy and textural attributes;

2. Identify the sequence of eruption of the volcanic rocks based on field occurrence, and;

3. Interpret the magmatic processes responsible for their formation.

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2 Review of Related work

Banahaw Volcanic Complex lies southwest of Luzon, east of Macolod. Holocene eruptions

of Mount Banahaw provided the sources for two youngest tephra layers: layers A (6 ka) and B (52

ka)3. which are compositionally made up of plagioclase and clinopyroxene. Mt Banahaw and its

surrounding areas are grouped into two morphological sectors. In the East, covering the beaches

of Mauban Quezon, is the Eastern Banahaw Sedimentary-Metamorphic Sector, while in the west

lies the Western Banahaw Volcanic Sector, which covers BVC4. The western volcanic sector is

further be subdivided into three major volcanic units based on the degree of erosion of the eruptive

centers, namely (1) Caliraya volcanic complex, (2) Malepunyo volcanic complex, and (3)

Banahaw volcanic complex5.

At least 4 distinct cone-building episodes are present in the Banahaw Volcanic Complex,

and each of this eruptive event comprise a separate volcanic group. . First is the Pinagdanglayan

volcanics, which is dominantly basaltic to andesitic tuff breccia. Second is San Cristobal Volcanics

pertaining to the most recent volcanic products of Mt. San Cristobal. The third volcanic group is

the Banahaw Volcanics, referring to the volcanic products of Mt Banahaw. The fourth volcanic

group formed the cone-complex of Mt. Banahaw de Lucban, which are collectively termed as

Lucban volcanics4.

In 1999, Banahaw de Lucban is identified as a Quaternary volcanic cone (Figure 2) that is

compositionally made of fine to medium grained andesite and/or basalt lava flow, overlying

Laguna formation6.

3 Methodology

With the aid of both megascopic and petrographic identification, the mineralogical

composition and textures of the collected samples were undertaken. Petrography also allowed the

determination of the order of crystallization exhibited by the rocks. These information paved way

for interpreting the magmatic history of the rocks, specifically, in Banahaw de Lucban. The

methodological framework used in the investigation and study is shown as Figure 3.

3.1 Materials

Compilation of existing reports from the previous works done in Mt. Banahaw de Lucban and

other related scientific papers in particular of topics regarding petrography are collected from

libraries

__________________________ 3 Ku Y.P., Chen C.H., Song S.R., Iizuka Y., Shen J.J.S., (2009)."A 2 Ma record of explosive

volcanism in southwestern Luzon: Implications for the timing of subducted slab steepening". An

Electronic Journal of the Earth Sciences, Volume 10, Number 6, 25 June 2009. 4 Tebar, H.J. and Ruiz, C.C., (1989) Lithostratigraphy and Fault Structure of Mt. Banahaw

Geothermal Prospect, South Central Luzon, Philippines. 5COMVOL (1981). Catalogue of Philippine volcanoes and Solfataric Areas. Dost-phivolcs.

Revised Jan 1997. 6 Mines and Geosciences Bureau, (1999). “Geologic Map of Lucban Quadrangle”, Sheet 3362III.

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libraries of Mapúa Institute of Technology (MIT) and Philippine Institute of Volcanology and

Seismology (PHIVOLCS). Additional journals and articles are obtained from EBSCO (Elton B.

Stephens Co.) Discovery Service of MIT Library and other internet sources.

Figure 3. Conceptual framework

3.2 Sample Collection

Sampling points were limited by accessibility and occurrence of outcrops (Figure 4). For

this reason, such undertaking concentrated mainly at the northern portion of Banahaw de Lucban.

Sampling point boundaries were determined in conjunction with the geologic map of the area,

covering barangay Samil in Lucban, Quezon and Barrio Taytay in Majayjay, Laguna.

3.3 Field and Megascopic Analysis.

Information of the sampling points were taken during the fieldwork. These information include

basic megascopic rock identification, elevation, and structure of the outcrop.

Table 1. Field and Megascopic Analysis

Sample No Elevation (masl) Structure(s) Present

Megascopic Rock Identification

BVS-JLR-01 750 Flow Basalt BVS-JLR-02 880 Flow Basalt BVS-JLR-03 1180 Contact and joints Basalt BVS-JLR-04 1400 (to 900) Flow Basalt BVS-JLR-05 1875 (to 1700) Flow Basalt BVT-JLR-06 850 Joints Andesite

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.

Figure 4. Sample location and outcrop images

Petrographic Analysis

Petrographic analysis of the collected samples at Banahaw de Lucban includes the

identification and percentage estimation of primary and secondary minerals. The textural attributes

are also identified since they are crucial in providing insights on the crystallization history of the

rocks. Plagioclase are analyzed through Michel Levy method to identify the anorthite content of

the plagioclase, which is needed to estimate the initial temperature in which the melt started to

crystallize. Plagioclase analysis is summarized on Table 2, while the all the petrographic findings

are summarized on the table 3.

4 Results/Discussion

The observed characteristics of plagioclase are summarized in Table 2, while the overall

petrographic attributes are condensed in Table 3.

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Table 2. Plagioclase Analysis

Table 3. Summary of Petrographic Analysis

Sample

No.

Primary Minerals Glass

Groundmass

Secondary Minerals An

%

Primary/

Flow Texture Rock Type Ferromagnesian

Mineral Plagioclase

Accessory

Minerals

Alteration

minerals

1 Augite 30% 40% 29% Magnetite

1% - 57

Porphyritic

Intersertal

Pyroxene

Basalt

2 Augite 35% 43% 20% Magnetite

2% - 59

Porphyritic

Intersertal

Pyroxene

Basalt

3A Augite 45% 45% - Magnetite

7%

limonite

2%

actinolite <1%

69 Cumulophyric

Intergranular

Pyroxene

Basalt

3B Augite 45% 45% - Magnetite

7%

limonite

2% actinolite

<1%

chlorite <1%

69 Cumulophyric

Intergranular

Pyroxene

Basalt

3C Augite 35% 43% 20% Magnetite

2% - 61

Cumulophyric

Intersertal

Pyroxene

Basalt

3D Augite 40% 50% - Magnetite

7%

limonite

2% actinolite

<1%

67 Cumulophyric

-Intergranular

Pyroxene

Basalt

4 Augite 35% 43% 20% Magnetite

%2 - 53

Porphyritic

Intersertal

Pyroxene

Basalt

5 Augite 10% 80% - Magnetite

8% - 59

Porphyritic

Orthophyric

Pyroxene

Basalt

6 Oxyhornblende

10% 25% 60%

Magnetite

5% - 28

Porphyritic

Pilotaxitic

Oxyhornblende

Andesite

Focusing on the observed attributes, the rocks can be characterized into four, namely:

orthophyric pyroxene basalt, intersertal pyroxene basalt, intergranular pyroxene basalt, and

oxyhornblende andesite.

Sample No. Maximum extinction

angle Anorthite percentage Initial Temp

BVS-JLR-01 34° 57 1290°C

BVS-JLR-02 35° 59 1300°C

BVS-JLR-03A 41° 69 1350°C

BVS-JLR-03B 41° 69 1350°C

BVS-JLR-03C 36° 61 1320°C

BVS-JLR-03D 40° 67 1340°C

BVS-JLR-04 33° 53 1280°C

BVS-JLR-05 35° 59 1300°C

BVT-JLR-06 19° 28 1180°C

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(1) Orthophyric pyroxene basalt (Sample 5), with coarse-grained euhedral to subhedral

plagioclase laths and medium-grained augite as phenocryst, with groundmass composed of fine-

grained orthophyric plagioclase. Accessory mineral magnetite occur in minor amount.

Figure 5. Thin section images of sample 5 identified as orthophyric pyroxene basalt. First and

second images (5x magnification under cross polarized light) display the coarse grained

plagioclase and augite phenocrysts embedded in a fine grained groundmass. Third image

(20x magnification under cross polarized light) displays orthophyric texture of the fine

grained plagioclase groundmass.

Petrogenesis: Orthophyric pyroxene basalt started to crystallize in a relatively lower temperature

at 1300°C (from anorthite percentage), though the magma contains high amount of feldspar, and

low amount of ferromagnesian as shown in their mineral percentages. Initially, the magma must

have been in a stable environment for large phenocrysts of plagioclase to form and grow, until a

disturbance, presumably an eruption, occurred, which caused the crystallization of the remaining

melt into the fine grained groundmass, subsequently forming embayment of large plagioclase by

the groundmass. The post-eruption temperature is low enough for small feldspar to form, but high

enough not to form glass. Additionally, the orthophyric (stumpy and idiomorphic) plagioclase

groundmass indicates that the remaining melt crystallized in a stable, equilibrium condition after

the presumed eruption.

(2) Intersertal pyroxene basalt (Samples 1, 2, 3C, and 4), with augite and plagioclase as primary

minerals and interstices occupied by glass and small crystal. Noted accessory mineral is magnetite.

Figure 6. Thin section images of samples 1, 2, 3C and 4 identified as intersertal pyroxene basalt

taken at 5x magnification under crossed polarized light. Images display augite phenocrysts

with 2nd order interference colors, embedded in the groundmass composed of plagioclase

and glass.

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Figure 7. Thin section images of samples 1, 2, 3C and 4 identified as intersertal pyroxene basalt

with particular focus on the groundmass. The groundmass is composed of plagioclase laths

and interstitial spaces filled with glass. Images are taken at 10x magnification under plane

polarized light.

Petrogenesis: The magma of intersertal pyroxene basalts started to crystallize in a relatively lower

temperature more favorable for pyroxene formation at around 1320°C (based from the anorthite

content of plagioclase). Subhedral phenocrysts suggests that the magma is not stable enough to

form euhedral crystals. An eruption caused a rapid decrease in temperature, which in turn caused

the remaining melt to rapidly form glass.

(3) Intergranular pyroxene basalt (Samplesd 3A, 3B and 3D), with euhedral to subhedral augite

and plagioclase as primary minerals in an intergranular groundmass composed of plagioclase laths

and interstitial augite, magnetite, and limonite-altered augite. Aside from limonite, other noted

alteration is actinolite and chlorite (on sample 3B), which is indicative of low grade

metamorphism.

Figure 8. Thin section images of samples 3A, 3B, and 3D identified as intergranular pyroxene

basalt taken at 10x magnification under cross polarized light. Images emphasizes on augite

phenocryst, displaying 2nd order interference colors. The cumulophyric characteristic of

augite is also exhibited. The groundmass is composed of smaller plagioclase laths and

interstitial magnetite and augite.

Figure 9. Thin section images of intergranular pyroxene basalt samples taken at 50x magnification

under plane polarized light. Images showcase secondary minerals of intergranular

pyroxene basalt: opaque minerals are secondary magnetite; brown minerals are limonite-

altered augite; and the colorless fibrous and radiating minerals are actinolite.

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Petrogenesis: The magma of olivine-pyroxene basalt started to crystallize at relatively higher

temperature around 1350°C (based from the anorthite percentage of plagioclase). A rather stable

environment aided in the formation and growth of euhedral and subhedral crystals which almost

impedes one another. Eruption of this magma causes the remaining melt to crystallize along the

interstices of the earlier formed crystals and create the characteristic intergranular texture. Presence

of actinolite and chlorite, minerals that are restricted to low-grade metamorphic rocks, is the result

of being overlain by a younger magma flow corresponding to the younger intersertal pyroxene

basalt.

(4) Oxyhornblende andesite (Sample 6), where the primary minerals are euhedral oxyhornblende

and plagioclase, embedded in glass groundmass. Flow texture of plagioclase microlaths are also

present. Noted alteration is the opacitization of oxyhornblende crystals into magnetite.

Figure 10. Thin section of sample 6 identified as oxyhornblende andesite. The phenocrysts consists

of oxyhornblende and coarse plagioclase laths, set in the groundmass compositionally

made of fine-grained plagioclase microlaths and glass. First and second image is taken at

5x magnification under plane polarized light and cross polarized light respectively. The

interference color (at crossed polars) of oxyhornblende is masked by its true color. In the

third image, taken at 10x magnification under plane polarized light, plagioclase microlaths

exhibit pilotaxitic texture.

Petrogenesis: The magma of oxyhornblende started to crystallize at around 1250°C (based from

anorthite content). Euhedral phenocrysts indicate stable temperature until a disturbance, probably

an eruption, occur. The glass groundmass and the alignment of plagioclase microlaths represent

this disturbance in terms of rapid decrease in temperature and flow respectively.

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5 Conclusions

Based on the results of the petrographic analysis aided by correlation of the outcrop

location, the following statements are concluded:

1. Most of the volcanic rocks collected are basalt in composition except for an andesite. The

composition of basalt is primarily augite and plagioclase whereas andesite is primarily

composed of plagioclase, oxyhornblende and glass. The abundance of opacitized

oxyhornblende in oxyhornblende andesite is caused by an exothermic reaction as it came in

contact with air or water during eruption7. The porphyritic texture and observed disequilibrium

textures such as phenocryst embayment and sieved plagioclase in all the sample are also

attributed to its eruption. On the other hand, the differences in mineralogy and textural

characteristics between the rocks led the characterization of the rocks into four:

(a) Intergranular pyroxene basalt, with characteristic actinolite alteration;

(b) Intersertal pyroxene basalt;

(c) Orthophyric pyroxene basalt, with high plagioclase percentage; and

(d) Porphyritic oxyhornblende andesite, with characteristic opacitization of

oxyhornblende.

2. Four eruptive events were identified based from petrographic analysis and field occurrence of

the volcanic rocks. Three eruptions are attributed to Banahaw de Lucban and one on Mt.

Banahaw.

(a) The oxyhornblende andesite magma may have originated from a much earlier eruption

of the main cone of Banahaw based from its exposure on the lower elevation masl

(850m) of Banahaw de Lucban in close proximity to the slope of Mt. Banahaw.

(b) The earliest eruption of Banahaw de Lucban is attributed to olivine - pyroxene basalt

outcrop present at the lower contact of sampling point 3.

(c) The second latest eruption is attributed to intersertal pyroxene basalt which overlies

olivine-pyroxene basalt at sampling point 3. Outcrop with this rock type are present

from 750 to 1400 masl.

__________________________

7 Ota R. (2008) “Oxidation of volcanic rocks”. The Journal of the Japanese Association of

Mineralogists, Petrologists and Economic Geologists, Vol. 41 (1957) No. 6 P 216-227.

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(d) The latest eruption is attributed to orthophyric pyroxene basalt present on the peak,

1875 to 1700 masl.

The volcanic rocks of the main cone predates the other volcanic rocks based on its

mineralogy and location. A cross section of Banahaw de Lucban (Figure 11) was modelled after

these inferred eruptions.

3. The three latest eruptions were identified as pyroxene basalt which has a mafic composition.

In contrast, the older oxyhornblende andesite in close proximity to the main cone is of

intermediate composition. For this reason, it is inferred that oxyhornblende andesite originated

from the youngest eruption of Mt. Banahaw, rather than from Banahaw de Lucban. On the

other hand, a younger, compositionally mafic magma source caused the eruption pyroxene

basalts of Banahaw de Lucban. In relation to this, the span of time between the eruptions of

pyroxene basalts must have been short, and probably be an episode of eruption from a single

eruption event. This episode of eruption is what caused the difference in texture among

pyroxene basalts

Figure 9. Cross Section of Banahaw de Lucban Modelled after Mapped Rock Units

Met

ers

abo

ve

Orthophyric pyroxene basalt

Intersertal pyroxene basalt

Intergranular pyroxene basalt

Oxyhornblende andesite

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References

1. Forster, H., Oles, D., Rnittel, U., Defant, M.J. and Torres, R.C., (1990). “The Macolod

Corridor: A rift crossing the Philippine island arc”. In: J. Angelier (Editor), Geodynamic

Evolution of the Eastern Eurasian Margin. Tectonophysics, 183: 265-271.

2. PHIVOLCS (Philippine Institute of Volcanology and Seismology). Volcano List-Banahaw

Volcano. Retrieved August 27, 2015 from

http://www.phivolcs.dost.gov.ph/html/update_VMEP/Volcano/VolcanoList/banahaw.htm

3. Ku Y.P., Chen C.H., Song S.R., Iizuka Y., Shen J.J.S., (2009)."A 2 Ma record of explosive

volcanism in southwestern Luzon: Implications for the timing of subducted slab steepening".

An Electronic Journal of the Earth Sciences, Volume 10, Number 6, 25 June 2009.

4. Tebar, H.J. and Ruiz, C.C., (1989) Lithostratigraphy and Fault Structure of Mt. Banahaw

Geothermal Prospect, South Central Luzon, Philippines.

5. COMVOL (1981). Catalogue of Philippine volcanoes and Solfataric Areas. Dost-phivolcs.

Revised Jan 1997.

6. Mines and Geosciences Bureau, (1999). “Geologic Map of Lucban Quadrangle”, Sheet

3362III.

7. Ota R. (2008) “Oxidation of volcanic rocks”. The Journal of the Japanese Association of

Mineralogists, Petrologists and Economic Geologists, Vol. 41 (1957) No. 6 P 216-227.