Download Classification of common igneous rocks: occurring in the Phil.

Classification of common igneous
rocks: occurring in the Phil.
processes: an increase in temperature, a decrease in pressure, or
a change in composition.
Morphology and setting
GEOLOGY AND PETROLOGY OF THE
MASINLOC CHROMITE
DEPOSIT, ZAMBALES, LUZON, PHILIPPINE
ISLANDS
W. C. STOLL
The Masinloc mine, the world’s largest producer of refractorygrade chrome ore,
is at Coto, Zambales, Luzon, Philippine Islands. Ore production
from 1946 through
1952 was 1,605,867.6 long tons, containing 32.33 per cent
Cr2O2. The ore occurs with
other chromite deposits in a layered ultramafic complex intruded
by microdiorite and
similar dikes and overlapped by Miocene sediments…
In terms of modes of occurrence, igneous rocks can be either
intrusive (plutonic), extrusive (volcanic) or hypabyssal.
Intrusive igneous rocks
Intrusive igneous rocks are formed from magma that cools and
solidifies within the crust of a planet. Surrounded by pre-existing
rock (called country rock), the magma cools slowly, and as a
result these rocks are coarse grained. The mineral grains in such
rocks can generally be identified with the naked eye. Intrusive
rocks can also be classified according to the shape and size of
the intrusive body and its relation to the other formations into
which it intrudes. Coarse grained intrusive igneous rocks which
form at depth within the crust are termed as abyssal; intrusive
igneous rocks which form near the surface are termed
hypabyssal.
Extrusive igneous rocks
Further reading:
http://geolsocphil.org/journal_abstracts/1962v16n1mar
_1abs.htm
GEOLOGY, STRUCTURE, AND ORIGIN
OF THE MAGNESITE DEPOSITS OF PISO
POINT,
MUNICIPALITY OF LUPON, DAVAO PROVINCE
L. SANTOS-YNIGO, M. R. LUCAS, and R. DE GUZMAN
The Lupon magnesitc deposits are concentrated at the
extremities of seaward-jutting spurs of an elliptical mass of
peridotite. Peripheral remnants of layered gabbro suggests an
original domal structure for the ultramafic complex, later messed
up during 2 separate periods of deformation, namely: (1) an
early period of thrusting movements, and (2) a later period of
strike-slip movements. The complex is cut by late gabbro and
anorthosite dikes which apparently came up during a quiet
interval between the 2 periods of major deformation. (Emphasis
supplied)
Basalt (an extrusive igneous rock in this case); light colored
tracks show the direction of lava flow.
Extrusive igneous rocks are formed at the crust's surface as a
result of the partial melting of rocks within the mantle and crust.
Extrusive Igneous rocks cool and solidify quicker than intrusive
igneous rocks. Since the rocks cool very quickly they are fine
grained.
The melted rock, with or without suspended crystals and gas
bubbles, is called magma. The volume of extrusive rock erupted
annually by volcanoes varies with plate tectonic setting.
Extrusive rock is produced in the following proportions:
•
•
•
divergent boundary: 73%
convergent boundary (subduction zone): 15%
Hotspot: 12%.
Hypabyssal igneous rocks
Hypabyssal igneous rocks are formed at a depth in between the
plutonic and volcanic rocks. Hypabyssal rocks are less common
than plutonic or volcanic rocks.
Further enrichment:
http://geolsocphil.org/journals.htm
Igneous rock may form with or without crystallization, either
below the surface as intrusive (plutonic) rocks or on the surface
as extrusive (volcanic) rocks. This magma can be derived from
partial melts of pre-existing rocks in either a planet's mantle or
crust. Typically, the melting is caused by one or more of three
1.
2.
3.
4.
5.
6.
7.
8.
use descriptive attributes;
use actual properties;
use current terminology;
define boundaries of rock species;
follow natural relations;
use modal mineralogy;
if mode not feasible, use chemistry;
Follow terminology of other IUGS advisory bodies.
PYROCLASTIC ROCKS AND TEPHRA
By defining the term "pyroclast" in a broad sense, the
classification can be applied to air fall, flow and surge deposits
as well as to lahars, subsurface and vent deposits (e.g.
hyaloclastites, intrusion and extrusion breccias, tuff dykes,
diatremes, etc.).
The terms used in the classification solely describe the
granulometric state of the rocks or deposits. Combined with
other terms, however, compositional or genetic information may
be included.
When indicating the grain size of a single pyroclast or the
middle grain size of an assemblage of pyroclasts the general
terms "mean diameter" and "average pyroclast size" are used,
without defining them explicitly, as grain size can be expressed
in several ways. It is up to the user of this classification to
identify the method by which grain size was measured in those
cases where it seems necessary to do so.
Pyroclasts
Pyroclasts are defined as fragments generated by disruption as a
direct result of volcanic action. Note that this excludes particles
formed by autobrecciataion of lava flows, because the flow itself
is the direct result of volcanic action, not its brecciation.
The fragments may be individual crystals, crystal fragments,
glass fragments or rock fragments. Their shapes acquired during
disruption or during subsequent transport to the primary deposit
must not have been altered by later redepositional processes. If
they have the fragments are called "reworked pyroclasts” or
“epiclasts if their pyroclastic origin is uncertain. The various
types of pyroclasts are mainly distinguished by their size.
•
Bombs – is a pyroclasts whose mean diameter exceeds
64mm and a shape or surface (e.g. bread-crust surface),
which indicates that they were in a wholly or partly
molten condition during their formation and subsequent
transport.
•
Blocks - pyroclasts whose mean diameter exceeds
64mm and who’s angular to subangular shape indicates
that they were solid during their formation.
•
Lapilli - pyroclasts of any shape with a mean diameter
of 64mm to 2mm.
•
Ash grains - pyroclasts with a mean diameter of less
than 2mm. They may be further divided into coarse ash
grains (2mm to 1/16mm).Fine ash grains (less than
1/16mm). The fine ash grains may also be called dust
grains.
Pyroclastic Deposits
Pyroclastic deposits are defined as an assemblage of pyroclasts
which may be unconsolidated or consolidated. They must
contain more than 75% by volume of pyroclasts, the remaining
materials generally being of epiclastic, organic, chemical
sedimentary, or authigenic origin. When they are predominantly
consolidated they may be called pyroclastic rocks and when
predominantly unconsolidated they may be called tephra.
The following terms are applicable to unimodal and well-sorted
pyroclastic rocks (Table B.1):-
Agglomerate – is a pyroclastic deposit whose average pyroclast
size exceeds 64mm and in which rounded pyroclasts
predominate.
Pyroclastic breccia-is a pyroclastic rock whose average
pyroclast size exceeds 64mm and in which angular pyroclasts
predominate.
Lapilli tuff - is a pyroclastic rock whose average pyroclast size
is 64mm to 2mm.
Tuff or ash tuff - a pyroclastic rock whose average pyroclast
size is less than 2mm. lt may be further divided into coarse (ash)
tuff (2mm to 1/16mm) and fine (ash) tuff (less than 1/16mm).
The fine ash tuff may also be called dust tuff. Tuffs and ashes
may be further qualified by their fragmental composition as
shown in Fig. B.1, i.e a lithic tuff would contain a predominance
of rock fragments, a vitric tuff would contain a predominance of
pumice and glass fragments, and a crystal tuff would contain a
predominance of crystal fragments.
Mixed Pyroclastic-Epiclastic Deposits
For rocks which contain both pyroclastic and normal classic
(epiclastic) material the Subcommission suggested that the
general term tuffites can be used within the limits given in Table
B.2. Tuffites may be further divided according to their average
grain size by the addition of the term "tuffaceous" to the normal
sedimentary term e.g. tuffaceous sandstone.
Further enrichment visit at:
http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGSIgneousClassFlowChart.htm#LamClass
PHILIPPINE GEOLOGY AND MINERALIZATION: AN OVERVIEW
The Philippines may be viewed as a collage of
metamorphic terranes, magma tic arcs, ophiolitic
complexes, sedimentary basins and continental block of
Eurasian affinity subjected to tectonic processes such as
subduction, collision and major strike slip faulting. The
subduction zones are represented on the east by the west
dipping Philippine Trench traversing the eastern seaboard
of the Philippines from Mindanao up to a point in Luzon
and the East Luzon Trough. East dipping subduction
zones include the Manila Trench, Negros Trench and
Cotabato Trench. The southern termination of the Manila
Trench is characterized by the transformation of the
subduction of the South China Sea Plate into an arccontinent collisional deformation within Mindoro Island.
The continental block is represented by northern Palawan,
southern Mindoro, Romblon Island Group and Buruanga
Peninsula in Panay Island, known collectively as the North
Palawan Block. Rock suites in this block include schists
that are characteristically rich in quartz and chart
formations that have been dated Late Permian to Jurassic.
The rest of the archipelago is considered as the Philippine
Mobile Belt. Approximately co-axial with the mobile belt is
the Philippine Fault, a major strike slip fault that apparently
developed partially in response to the kinematic forces
from the subduction from the east and west of the mobile
belt.
Many areas of this mobile belt are underlain by ophiolitic
complexes. Usually occurring together with pre-Cenozoic
schists and phyllites, the ophiolitic rocks represent
basement on which magma tic arcs have developed. The
ages of the ophiolitic complexes range from Jurassic to
early Paleogene. One of the best studied complete
ophiolite sequence is the Zambales Ophiolite where
tectonized peridotites progress to layered and isotropic
gabbro, sheeted dike complex, pillow basalts and finally
pelagic sedimentary rocks. Other ophiolitic complexes
include those in Isabela, Polillo Island, eastern Rizal,
Camarines Norte, Caramoan Peninsula, Mindoro, southern
Palawan, Panay Island, Bohol Island, Leyte Island, Samar
Island, Dinagat Island Group, north-central Zamboanga,
Mindanao Central Cordillera and Pujada Peninsula.
Ultramafic rocks of these ophiolites are hosts to significant
deposits of chromite and nickel. Laterites over these rocks
also contain economic deposits of secondary nickel
minerals. On the other hand, massive sulphide and
manganese deposits are associated with the volcanic and
sedimentary carapace of the ophiolite.
Ancient magma tic arcs in the mobile belt are
characterized by thick volcanic flows intercalated with
pyroclastic and sedimentary rocks and intrusions of diorite,
quartz diorite and andesitic to dacitic rocks. Some
intrusions, however, have a more alkalic character such as
the syenites in Isabela and monzonites in Quirino and
Nueva Vizcaya. The ages of the diorite intrusions vary,
from late Early Cretaceous (Albian) in Cebu to Late
Miocene-Pliocene (Black Mountain Quartz Diorite in
Baguio District). Younger volcanic rocks, occurring as
flows, intrusions and volcanic edifices disposed in linear
belts are associated with active subduction processes.
These are best exemplified by the Bataan volcanic belt
and Bicol volcanic chain.
Sedimentary basins located between arcs include the
Ilocos-Central Valley Basin, Cagayan Valley Basin,
southeast Luzon Basin, Visayan Sea Basin, AgusanDavao Basin and Cotabato Basin.
Gold and copper deposits in the Philippines tend to be
clustered in certain areas such as Luzon Central
Cordillera, Camarines Norte, Surigao and Davao, although
large deposits may also be found elsewhere, as in
Zambales (Dizon mine), Cebu (Atlas mine) and South
Cotabato (Tampakan project). Many copper-gold deposits
are associated with intrusions (mostly diorite and quartz
diorite, but also monzonites and syenites) as well as
Pliocene – Pleistocene volcanism (Lepanto mine at
Mankayan, Benguet). Iron deposits are also associated
with Neogene intrusions of diorite and quartz diorite.
(Emphasis supplied)
Reference:
http://philippinestamps.net/RP2009-Minerals.htm
Identification of Igneous Rocks
Grain
Size
Usual
Color
Other
Composition
Rock
Type
fine
dark
glassy appearance
lava glass
Obsidian
fine
light
many small bubbles
lava froth from sticky lava
Pumice
fine
dark
many large bubbles
lava froth from fluid lava
Scoria
fine or
mixed
light
contains quartz
high-silica lava
Felsite
fine or
mixed
medium
between felsite and basalt
medium-silica lava
Andesite
fine or
mixed
dark
has no quartz
low-silica lava
Basalt
mixed
any color
large grains in fine-grained
matrix
large grains of feldspar, quartz, pyroxene or olivine
Porphyry
coarse
light
wide range of color and grain
size
feldspar and quartz with minor mica, amphibole or
pyroxene
Granite
coarse
light
like granite but without quartz
feldspar with minor mica, amphibole or pyroxene
Syenite
coarse
light to
medium
little or no alkali feldspar
plagioclase and quartz with dark minerals
Tonalite
coarse
medium to
dark
little or no quartz
low-calcium plagioclase and dark minerals
Diorite
coarse
medium to
dark
no quartz; may have olivine
high-calcium plagioclase and dark minerals
Gabbro
coarse
dark
dense; always has olivine
olivine with amphibole and/or pyroxene
Peridotite
coarse
dark
dense
mostly pyroxene with olivine and amphibole
Pyroxenite
coarse
green
dense
at least 90% olivine
Dunite
very coarse
any color
usually in small intrusive
bodies
typically granitic
Pegmatite
Reference:
http://geology.about.com/library/bl/blrockident_tables.htm
How to Look at a Rock
By Andrew Alden
People don't usually look at rocks closely. So when they find a stone that intrigues them, they don't know what to do,
except to ask someone like me for a quick answer. After many years of doing so, I hope to help teach you some of the
things that geologists and rockhounds do. This is what you need to know before you can identify rocks and give each one
its proper name.
Where Are You?
The first thing I ask a questioner is, "Where are you?" That always narrows things down. Even if you aren't familiar with
your state geologic map, you already know more about your region than you suspect. There are simple clues all around.
Does your area contain coal mines? Volcanoes? Granite quarries? Fossil beds? Caverns? Does it have place names like
Granite Falls or Garnet Hill? Those things don't absolutely determine what rocks you might find nearby, but they are
strong hints.
This step is something you can always keep in mind, whether you're looking at street signs, stories in the newspaper or
the features in a nearby park. And a look at your state's geologic map is intriguing no matter how little or how much you
know.
Make Sure Your Rock Is Genuine
Make sure you have real rocks that belong where you found them. Pieces of brick, concrete, slag and metal are
commonly misidentified as natural stones. Landscaping rocks, road metal and fill material may come from far away. Many
old seaport cities contain stones brought as ballast in foreign ships. Make sure your rocks are associated with a real
outcrop of bedrock.
There is an exception: many northern localities have lots of strange rocks brought south with the Ice Age glaciers. Many of
the state geologic maps show surface features related to the ice ages.
Now you will start to make observations.
Find a Fresh Surface
Rocks get dirty and decay: wind and water make every kind of rock slowly break down, the process called weathering.
You want to observe both fresh and weathered surfaces, but the fresh surface is most important. Find fresh rocks in
beaches, roadcuts, quarries and streambeds. Otherwise, break open a stone. (Don't do this in a public park.) Now take
out your magnifier.
Find good light and examine the rock's fresh color. Overall, is it dark or light? What colors are the different minerals in it, if
those are visible? What proportions are the different ingredients? Wet the rock and look again.
The way the rock weathers may be useful information—does it crumble? Does it bleach or darken, stain or change color?
Does it dissolve?
Observe the Rock's Texture
Observe the rock's texture, close up. What kind of particles is it made of, and how do they fit together? What's between
the particles? This is usually where you may first decide if your rock is igneous, sedimentary or metamorphic. The choice
may not be clear. Observations you make after this should help confirm or contradict your choice.
•
•
•
Igneous rocks cooled from a fluid state and their grains fit tightly. Igneous textures usually look like something you
might bake in the oven.
Sedimentary rocks consist of sand, gravel or mud turned to stone. Generally they look like the sand and mud they
once were.
Metamorphic rocks are rocks of the first two types that were changed by heating and stretching. They tend to be
colored and striped.
Observe the Rock's Structure
Observe the rock's structure, at arm's length. Does it have layers, and what size and shape are they? Do the layers have
ripples or waves or folds? Is the rock bubbly? Is it lumpy? Is it cracked, and are the cracks healed? Is it neatly organized,
or is it jumbled? Does it split easily? Does it look like one kind of material has invaded another?
Some kinds of structural features, like concretions, folds, ripples and slickensides, appear in this gallery of geologic
features and processes.
Try Some Hardness Tests
The last important observations you need require a piece of good steel (like a screwdriver or pocket knife) and a coin. See
if the steel scratches the rock, and then see if the rock scratches the steel. Do the same using the coin. If the rock is softer
than both, try to scratch it with your fingernail. This is a quick and simple version of the 10-point Mohs scale of mineral
hardness: steel is usually hardness 5-1/2, coins are hardness 3, and fingernails are hardness 2.
Be careful: a soft, crumbly rock made of hard minerals may be confusing. If you can, test the hardness of the different
minerals in the rock.
Now you have enough observations to make good use of the quick rock identification tables. Be ready to repeat an earlier
step.
Observe the Outcrop
Try to find a larger outcrop, a place where clean, intact bedrock is exposed. Is it the same rock as the one in your hand?
Are the loose rocks on the ground the same as what's in the outcrop?
Does the outcrop have more than one kind of rock? What is it like where the different rock types meet each other?
Examine those contacts closely. How does this outcrop compare to other outcrops in the area?
The answers to these questions may not help in deciding on the right name for the rock, but they point to what the rock
means. That's where rock identification ends and geology begins.
Getting Better
The best way to take things further is to start learning the most common minerals in your area. Learning quartz, for
instance, takes only a minute once you have a sample.
A good 10X magnifier is worth buying for close inspection of rocks. It's worth buying just to have around the house. Next,
buy a rock hammer for efficient breaking of rocks. Get some safety goggles at the same time, although ordinary glasses
also offer protection from flying splinters.
Once you've gone that far, go ahead and buy a book on identifying rocks and minerals, one you can carry around. Visit
your nearest rock shop and buy a streak plate—they're very cheap and can help you identify certain minerals.
At that point, call yourself a rockhound. It feels good.
Jedi- yah, it feels good.